It's worth it if they enjoy doing it at the time
Hmm. See, my boys find piano hard. They don't really enjoy practising; and when they are learning a new piece, the struggle can make them quite cross and upset. Neither of them, I am convinced, will end up a pianist, and we are all happy with that.
But being able to play - that, they enjoy. And every time they make a leap, that's a delight for them.
And, frankly, they don't find much else they are exposed to at the moment especially hard, nor get the corresponding thrill from having mastered it. I think that matters. I realise that piano is not the only way to counter this.
It's an interesting balance for me, as a parent - and also, as a human being for whom music is really important, and who regrets never having really put the effort in to be a properly competent player.
I won't force them if it is making them properly miserable - of course I won't. But at the moment, at age 12... I am not letting them give up, even though it often makes them cross and grumpy, and even though I could really, really use not spending the money. Because I honestly think that it's "paying off", for them.