It's exactly true that you don't need all that penetration to counter a P4, and that's not why you buy something like a Jackson. You get one to actually be able to deal with Panthers and up, which it should absolutely be effective at.
ATGs aren't going to stop a Panther unless you have several of them, and they're certainly not going to kill a fleeing or blitzing one. AT infantry, maybe, if you can snare them at the right time and place, but again, even Bazooka rangers aren't going to be chipping through all that armor and hp anytime quickly.
In all the TD complaint threads before this, people were complaining that allied TD dominance erased Axis MTs from play. Reframing the issue in the context of being too effective versus massively armored endgame axis tanks is particularly stupid - it just sounds like you want these tanks to be an I-win button.
TDs should absolutely counter panthers, they should absolutely counter heavy tanks. Especially in the age of high AOE heavy tanks blasting apart ATGs, which are more vulnerable than ever at the end stage of the game, this is no time to be nerfing TD penetration of all things, buffing heavy tanks even more than they already are.
Except that Panther's are at least Ostheer's equivalent of a heavy TD, and Ost relies heavily on vehicles to provide its anti infantry. If you can shut down ostheers anti infantry with TDs, and ostheer's primary counter to armor(panther) is hard countered in the same way, then you are left with a faction that struggles too heavily against certain stock end game compositions.
The divide between lighter and stronger AT (m10 vs jackson, su76 vs su85) should be one of DPS vs penetration if the current penetration vet is to be maintained. If the SU85 is allowed to retain near 100% chance to penetrate axis heavies at vet 3, then it should have its DPS reduced.
The panther does NOT need this for two reasons, one because it doesn't recieve a penetration bonus with vet anyway, and two because it costs far more supply than allied TDs, the DPS/per supply is quite a bit lower which is important when considering what an endgame composition looks like for their respective factions.
I don't really see how you can reasonably argue for the massive penetration boost the Su85 and the Jackson get with vet. You cannot ignore how disgusting it is a vehicle like the Jackson boasts 286 long range penetration (0% chance to bounce on panther) with vet. It negates the importance of armor as a stat and makes HVAP rounds completely redundant.