You have to agree that SU-76 is too good for its cost though. There's no incentive to build SU-85 at all when you can just sit on SU-76s (which are also way easier to vet up). SU-76 should be a medium tank destroyer and SU-85 the heavy tank destroyer.
I'd say that is the problem of SU-85, which got "penal battalion" disease, where cheaper unit does exactly the same what it can do. Medicority of SU-85 in comparison to any other T3 level TD is the problem, StuG is cheap, JP4 have awesome scaling and durability, firefly have alpha strike, Jackson have mobility and reliability and SU-85 is just tad more dmg, tad more durability SU-76 with low mobility.
All TDs got updated and SU-85, just like penals in comparison to cons, lags behind because there is something that arrives earlier and does the same thing.
Its not that SU-76 is too strong.
SU-85 and penals are simply outdated, don't bring anything more to the table then their cheaper alternatives, arrive later and have less utility.
Just like nerfing cons all of sudden won't make penals more appealing, changing SU-76 won't make SU-85 more desirable.
KT is still a tank.
Both SUs are tank destroyers.
Tank destroyers damage and kill tanks.