The way you word it it sounds like the StuG is the reason people dont build Panthers. But even if there was no StuG in the game, I wouldn´t build the Panther.
If there was not stug in the game Wer would be unplayable, one would never reach Tech 4 and thus one would never built a Panther...
The Stug is simply too cost efficient and it can even fight super heavy tanks. Direct result of that is that Panther has not been looked at although it has become cost inefficient with the arrival of m36s and fireflies that can penetrate and kill it with 4 shots.
So in a sense yea the performance of the stug is directly related with the state of the Panther.
If you are interested you can find more about my opinion here:
https://www.coh2.org/topic/57834/kyle-wants-feedback-on-the-piv/page/3
In Relic's effort to provide counters to they Super heavy Axis vehicles, axis mediums become cost inefficient...