I am under the impression that it see less action than it used to before it was patches
Flame vehicles and units with ballistic guns work differently and each has its advantages and disadvantage. They are different and they are map depended than anything else.
Oddly that was the path that was chosen by the same MOD team for hezter although the PzIV already has excellent AI.
I don't think Sander needs explaining how different weapon types work, mate... And by saying that Hetzer and KV8 got different treatments, you imply that the factions they are in have similar units. That's just wrong.
Your proposal would not change anything on map dependency. Downscaling cost and performance just clutters Soviets comparatively light tanks. At the moment it is a beefy AI specialist, something that Soviets lack in their stock rooster. Just like Soviets lack anything beefy in their stock rooster, that's why literally all their doctrinal tanks (maybe with the exception of the lend lease Sherman) are heavier than anything they could usually field.
The Hetzer just collides somewhat with the Luchs, a unit that OKW does not even have if they go for T1. OKW already has a beefy tank unit with okay AI (the P4 as you said), so why move the Hetzer into the same spot?
Of course the unit can still work like that, but giving them a distinct role is just the straight up better option.