Its like a vicious circle, allies build less mediums the bigger the game goes cuz of the panther hard countering those, hence there is no point in building japzr4 because allies go td or heavy because of the panthers.
A few years back panther had low rof of about 9 seconds to have it be less good vs mediums. Now its rof is about medium tank rof and its good at countering both heavy and medium armour, and jagpz4 got pushed aside.
That is no very accurate description:
A) JP was moved from T1 to T4 without nay discount and has received several nerfs since then
B)OKW had 6.3 reload not 9 but it also used to have 0.65 moving accuracy an used to gain range with vet.
C) M36 become better vs mediums with the higher ROF and SU-85 become better vs heavies
So yea it is vicious circle but not exactly the way you mean.