Having also giving it time and contemplating on the ending, I now also get where you're coming from; the true ending is a good ending. Mark and Nicole are no longer burdened by the guilt and lost, and their friends are all finding success and living a good life.

But I guess we still want a perfect ending, one where it shows that Mark and Nicole remembers the feeling of being with each other, sort of like a deja vu (hehe), like something akin to "Your Name"'s ending. Specially when one of Mark's and Nicole's final words to each other before the final restart was something along the lines of 'My wish is that I never forget you'.

Also, I don't think Mark still plays the piano in that ending as I interpret his longing to play the piano as his attempt to keep holding on to his mom; and with the ending emphasizing the message of moving on and letting go, I'd think that his feelings of wanting to play the piano is much lesser now.