As I read people lament the futility of building Zerglings against Hellbat-containing mech compositions in the HotS beta, I came to a staggering realization; Zealots don't fare much better, especially in lategame when mech upgrades come into play and become strong. It makes TvP mech much more than viable. It completely flips the matchup on its head and changes, even destroys some very fundamental and delicate interactions and dynamics which were never tampered with in WoL, as there were no complaints. The Hellbat's damage output has a disrupting effect on these fragile elements of the matchup, to the point where a composition of Seige Tank + Hellbat + Viking, a handful of Thors and some widow mines (and armory attack upgrades) has virtually no counter that I have found so far out of the combination of everything the Protoss army can field.
To give some idea of what I mean in the form of an illustrative (if lacking in numbers and statistics) anecdote: I was playing a PvT against a player with 8 or 10 or so Blue Flame Hellbats with +2 attack, I was on 4 bases pushing 5, having just taken my 5th and trying to defend it against the 3 base Terran pushing to take a 4th. I had 22 WarpGates and a +3/+3 maxed army (Colossi, Tempest, Storm, Mothership, Archons, the whole bit). I took an admittedly bad engagement (although I doubt if I could have successfully engaged the army despite being maxed out and on very good upgrades and production) and lost the bulk of my army although not all of it outright, but the scary part was what happened next, when remax/reinforcements normally come into play.
With 5 Nexuses and 22 WarpGates, I was warping in rounds of 22 3/3 chargelots, chronoboosting my WGs and attacking his Tank + Hellbat army with tons of Chargelots + Archons and whatever else I could muster on quick remaxes off of 3 Stargate 22 WG 2 Robo. Even with the tank count whittled away to almost nothing (at the cost of all of my Colossi and Tempests), he stood with Hellbats and a lot of useless non-landed Vikings.
My chargelots absolutely MELTED, to the point where they not only never closed the distance to his army to even land a single hit in melee (these are 3/3 chargelots and I had 22 Warpgates, remember), but the Hellbats literally melted the Zealots so quickly that the combined firepower of his remaining army even had enough time to make sure my Archons and Immortals never even made it into the battle, either.
I had a giant bank at this point, but I simply didn't know what to do. Colossi and Tempests weren't an option because of the air dominance provided by Vikings hard countering both, and the same goes for Mothership. HTs weren't viable because getting a storm off simply couldn't be done; I couldn't walk them up because Seige Tanks obviously have greater range than storm and walking HTs towards mech is insane, and putting them in a Warp Prism would just be food for the Viking army. This leaves me with no ranged splash damage, which isn't as important against mech as it is against bio but still is important given the nature of Hellbats and the stationary way the Terran mech army works.
So, with a lead in upgrades and one base up as well as a production advantage and access to the entire tech tree, I found myself out of options. Perhaps a tech switch into pure Immortal off of something insane like 5 Robo or so is/was the only thing that could have possible worked. Blink Stalkers were absolutely wrecked even when completely massed to the tune of 100+ supply worth and blinked at the mech army; somehow, widow mines + hellbats + tanks + a few thors make them melt up close pretty much as bad as if you try to walk them up and engage a tank line in a straight up firefight. Air units/Colossi are simply not an option (Vikings). High Templar can't close the distance. Zealots will never get to melee, even with 125 supply worth and max upgrades, that much is a fact. Archons fare better, but surprisingly, not much considering the insane cost difference.
A fully realized Terran mech death ball with upgrades consisting mostly of Seige Tanks and Hellbats supported by a handful of Thors and a number of widow mines and backed up by a large force of Vikings (to deny any possibility of Carriers, Colossi, Tempest, or a Mothership Vortex) is incredibly powerful, and handily defeats and Protoss max within reason.
Yes, theoretically, maxing out army supply on pure Archons (roughly 1500/9600), or pure Immortals (roughly 8000/3200) or such things would do the trick, but this simply isn't realistic. It's not realistic to ever expect to have such a lead that your gas income allows you to produce 32 Archons (supply cost based on an assumed 72 probe count for simplicity's sake), which is obviously 64 High Templar. That's basically 10,000 gas! That makes it a prohibitively gas-intensive army with literally no mineral sink save for Photon Cannons. And while Immortals are not terribly cost-prohibitive, the required production infrastructure seems to be, unless Protoss players are just conditioned to expect to be able to deal with any threats with 2 or a maximum of 3 Robotics Facilities, because of the way in which Robo and Stargate units act as supplementary to Warpgates and, correspondingly, these units have much longer production times than Protoss is designed to work with. Maxing out or nearly maxing out on Immortals, while not as unrealistic cost-wise (with a roughly 8 to 3 mineral to gas ratio) as the Archon example, is restricted by the infrastructure that Robo units are based and balanced around. A few Immortals, Chronoboosted out of a Robo, can be effective at helping a Protoss army to attack a position protected by seiged up tanks or other armor; a couple of Colossi out of a Robo, after a period of time and no small amount of Chronoboost, can again help a Gateway unit-based Protoss army against bio, meaning the difference between losing the battle in a very one-sided steamroll affair and actually pushing back, even outright killing a very similar bio army.
The problem lie in the operative word; help. Robo units are extremely powerful, and as such they are balanced to be firepower/toughness-based "support units"; extremely strong units adept at dealing with a specific unit type that a versatile Protoss army simply doesn't have the power to fight, but too strong to allow massing of on such a scale that it becomes the basis for an army in the same way that Terran chooses between bio and mech. Basically, Protoss is sometimes said to be fielding a "Gateway army", but in truth, the game design does not allow for Protoss to truly field a "Robo army". The build times are too high and the structure itself costs slightly too much. These units are simply not designed to exist as the primary building blocks of an army, the game isn't balanced that way.
So what happens when a Robo unit, such as the Immortal, is the only effective counter to a certain unit composition like HotS mech to the extent that it needs to be the primary unit in the Protoss army to combat the enemy composition, and any other units are superfluous if they are not both small in number and dedicated to a very specific role, i.e. giving your army some form of anti-air? What if, to combat this new brand of mech, Protoss requires a "Robo army", an army primarily of Immortals numbering in the dozens instead of merely 3 to 5, to soak up the massive amount of damage coming its way?
Clearly this should not actually be the case, but I'm concerned that A) It might actually be the case, and B) If it is the case, what is the answer to the problem? Either Blizzard needs an answer in the form of redesigning the Protoss army to deal with mech or doing something to allow Immortal production on the scale necessitated by however powerful mech ends up being, or the player needs an answer that does not currently exist, in the form of a way of sustaining such a high level of Robotics Facility production while not being completely naked and defenseless in the face of the most minor of tech switches, not merely to SkyTerran, but to something as simple as bio, in which case a maxed out Protoss on a half-dozen Robos but a comparatively small WG count would be, in such a late stage of the game, totally defenseless.
If the Hellbat-Seige Tank-Viking composition becomes half as powerful a 1-2-3 punch against Protoss as I suspect that it might, I don't think a mobile Protoss army will even be able to deal with it by abusing its immobility the way it is supposed to deal with something like Broodlord/Infestor in the current metagame; Terran has far better defensive and anti-harass properties than Zerg, especially in the latter phases of the game where they can afford Planetary Fortresses, a healthy contingent of missile turrets, and Seige Tanks here and there "just in case".
Taking on such powerful high-damage, high-durability units is the Immortal's role in the Protoss army. Ideally, this is a niche. But what actually happens when this former "niche" actually comes in the form of an entire army with not one but many high-damage, tough units, one to deal with each possibly existing unit type? The rock, the paper, and the scissors could all end up on one hand, giving no realistic and cost-effective counter (taking into account time as a cost, as well we should). Can the Immortal really be expected to be one unit to counter an entire diverse army just because they are "armored" or "mech"? The answer must lie in either adjusting the damage of this army to allow more of the "glass-cannon" type units to get some shots off before they are shattered, or in adjusting thing a bit on the Protoss end to give the bulk portion of their army some options/tools so they can stand up to the DPS of mech long enough to damage it rather than dying before ever closing the distance and dealing a single blow. Ideally, you never want to actually engage a full mech army in a seiged-up position, but in many instances, it just might become necessary, as space controlling tools allow Terran to force that hand on Protoss, whether they like it or not. Maps are only so big, and eventually the armies have to collide in some way lest one or both players be starved out, and if "going around" (initiating a base trade of sorts) is the only viable option, it will be overcompensated for because of that fact and it will cease to be an option; the armies will eventually have to fight. Again, I'm not contending that a Protoss army should be able to repeatedly butt heads with a mech army and kill it, merely that if/when push does come to shove, that they should stand somewhat more of a fighting chance than they currently do so as to make the battles less lopsided and increase the likelihood that a player with superior economy and production will win by attrition rather than spend through five times his opponents' bank remaxing only to melt to the very same army until he is broke and conceded the game.
I think the core problem, at least if there truly is one (and I suspect that there is or will be as the metagame evolves) lies in reinforcement cost & time. A Protoss in a superior position could trade against a very strong Terran army in Wings of Liberty and eventually come out on top by way of attrition on the back of their large bank and strong economy feeding Warp Gates which pumped out Chargelots to deal with ground units, Archons to tank damage, and Stalkers for anti-air, and these Gateway-based remax armies were enough to "mop up the mess" after a massive army trade, and if the game dictates that the trade has to be unfavorable, that was acceptable as long as Protoss had the economy to overpower his opponent through attrition. The Chargelot was the crux of this dynamic, and by handing Terran mech such a clear-cut hard counter to it in the Hellbot (which, make no mistake, is at the extreme end of the hard counter spectrum) the Zealot is rendered obsolete in the lategame, leaving Protoss with no solid cost-effective unit to reinforce with and ensuring that if Protoss has to remake their army against mech, that army will be solidly manhandled by superior units designed to counter them.
I feel that the Hellbat being effective against chargelots is not the problem, but rather that the degree in which they are effective is the real problem. They should help a mech army to better deal with chargelots, but they should not simply win over them by default, rendering them dead on arrival simply because of their presence in a given unit composition. This type of super-hard-counter, which is VERY rare in SC2, undermines almost every strategic aspect of the game, negates a very important part of the way Protoss armies leverage economic advantages over Terran, and all but eliminates the concept of attrition in the later stages of the PvT matchup, further contributing to the much-maligned "deathball" mentality, in which the winner is simply the player who can muster the "ultimate army", with nothing to fear from the player with the ever-so-slightly inferior army but far superior economy, production, and infrastructure. A restructuring of the way the Hellbat operates in PvT and the effects it has on the matchup in general would be a service to everyone who plays this great game and all those who take great joy in spending their time and money watching them do it.