I have enough faith in Heejae to say he’d 100% still stick with Garam. Their relationship has gone way beyond just being able to understand each other through music, they know each others souls now. Ofc music was what got them here but even if Garam could never play again i dont think Heejae would suddenly stop loving or depending on him. (Even in the chps heejae misses Garam not the sound of Garam's piano)
I don't think their relationship is simply tied to the whether or not Garam is able to play piano, it's in their ability to understand each other's soul so easily. Whether that's through music or not, I think they'll be able to express it in another other way should it happen, so yeah (In Garam's case at least, Heejae's another Pandora's box when it comes to music)

If Garam broke his hands one day and could no longer play piano, would Heejae still be this obsessed with him or would his interest eventually fade because he couldnt see his soul the way he used to