I think they are afraid of creating a tank that literally does everything. If say, the panther was really amazing at killing tanks right? But, it does some light anti infantry damage as well. If you get 2-3 panthers then the game is over. Nothing could stand up to that anymore. It's the same reason the comet got nerfed over and over again after release. Cuz the comet covered all bases.
2 panthers is 350 fuel and 3 panthers is approaching 525 fuel. Think about that level of fuel investment for a moment when considering their performance.
Aa far as the old comets, their main gun at the time would kill 1-2+ models per shot at maximum (sight) range. 2 comets could wipe an ostheer squad nearly everytime they saw one through FOW. Panther mgs require facing, don't function great at range, can't be given targets, and iirc comets also eventually get to throw grenades which is great at clearing atgs.