This guy was in pretty much constant pain from the limitations of his body but it never stopped him from playing.

https://www.youtube.com/watch?v=6PyYcnXQZJY&list=RDzWmz5-NoF...

Of course, better not to form bad habits, but your body has some pretty good signals to tell you that you are doing something wrong.

I'm not a classicist in fact. If you're Thelonius Monk go ahead and play, but in this same thread we have some people complaining of finger fatigue and cramps. That's something that a teacher can correct easily.

No, because the piano is not an ergonomic instrument. No matter what, you're going to experience finger fatigue and cramps. And that's even assuming you have a hand size that fits the one, standardized keyboard size.