The high speed of the Panther is off set by it's awful accuracy while on the move. Meaning you need to stop before each shot in order to reliably hit your target. Ostheer is also a defensive oriented faction meaning the high mobility of the Panther doesn't make sense when you will normally not be chasing your enemy.
Non-Panther tanks wouldn't become invalid at all, the OKW PIV is way, way more popular than the OKW PV. Basically if given the choice of generalist or specialist most people will continue to pick generalist.
The high pen but very low ROF is actually great for Allied tanks because it allows them to overwhelm without taking significant losses.
Not as long as the Soviet T4 is equivalent to Ostheer T3, you're not supposed to get Panthers easily.
Ostheer T3 doesn't include rocket artillery. Perhaps the fix here might be moving the Pwerfer to T3?
Yes it is. Or at least it should be. Otherwise there's no intensive to use anything else. That's all the Alpha was, Comet spam, and that's why people didn't realise how fucked up the Churchill was.
Thing is, UKF, USF, Soviets, and OKW all have the ability in a match to access their entire unit tree but Ostheer for some reason is the exception. The whole "bonus tier" myth makes no sense because nothing in Ostheer T4 is actually more powerful than what other factions get over the period of a normal game anyway.
Everything in T4 is expensive, you won't be seeing any Panther or Sturmpanzer IV spam even if you make T4 cheaper to acquire.
Regardless Ostheer T3 is suffering heavily now because by the time it hits the field it's been countered to death.