Yes, what Vipper's saying is that for TDs like the SU85 and JP, that can't chase mediums and be circle strafed, you need a decent medium tank, e.g. the PzIV, to face other mediums.
What I am saying is that JP issue has very little do with Panther. Panther could made to be less effective vs mediums and people would still not build JPs when facing mediums.
An okw player has little reason to go JP if his facing allied mediums, since it cost is much higher then them and PzIV J is good enough with a similar and with AI.