I've always hated the "use a panther arguement". The panther has the penetration yes, but it doesn't have the RoF to take down 1400HP worth of churchill or even the IS2 without support showing up from your opponet. The panther and jp4 are both great vs mediums, but neither are stellar vs heavies. If we have the diversity to make both, why not specialize them. Taking down IS2s frontally is even a challenge for a panther given RNG is semi reasonable. The jp4 also wouldn't struggle as much if its veterancy was fixed and the IS2 armor was lowered.
The use panther argument is there because that's literally why the panther is there.. To fight heavy armour. Any issues with it fighting a literal meatshield lies with the balance of the meat shield (side note, if the panther didn't struggle to grind down 1400hp designed solely to be a pain in the ass to grind down what would be the point? What's more,the panther would then absolutely obliterate anything with less than 1400hp far too easily. That would create a broken mirror to allied TDs being buffed to laugh at the KT and ignoring the balance implications of units with less armour than the KT.
Keep in mind the is-2 was designed in game as the only unit that can really fight the panther, and is limited to 1 only. It's a better tank due to restrictions. It wasn't always.
The JP4 isn't designed to tackle the most armored unit in the allied lineup. If it was capable of doing that we alwould see the same thing we have with allied TDs which is BAD design, not better design. The JP4 excels at munching mediums and can actually fire back at allied TDs. That's its job, not heavy armour.
If we can move some power away from allied infantry and TDs so that mediums see more play we will see an up shoot in viability to no panther options. The issue is that allied infantry provide enough AI that there is no need to get a medium when you can get an omnipotent TD that will ensure that enemy armour can't level the playing field.