Panthers are far from no-brainers. Building a panther literally means no brains. Why would you ever build a unit that is too expensive to tech, too expensive to build and even takes too much population
Just because panthers are survivable that dosen't make them good tanks. It's suprisingly hard to kill anything with them. They have slowihs rof and they miss from the move often. Panthers get usually pushed back by at guns before they can kill their targets.
the panther have an excellent combination of armor, hp, and speed that given it excellent survivability. It can outrun anything it can't outfight and outfight anything it can't outrun. It also have 50 range meaning it can out range anything it can't outfight.
If it have good dps on top of all those advantages it would be unbeatable.
yes, the panther is not perfect, and that's the point. If there's a unit that's truly perfect with no weakness then it would be overpowered.