I think there’s something different than this: The relation of the members. Yoshiki said that he’s in America for 14 months, I wonder if they have called each other even once or not during that time. It seems like members are not close that much.
If it were only about the Yoshiki's desire about being an only-pianist and his fear of the getting injured again, they would have a concert least 20 times among this three years.
We're already know that when he wants to do something, then he gets it in any case.
If the problem were only this, then they would have acoustic concerts. So he wouldn't have to be playing drum and only play song with the piano.
Or they would have a concert which includes hard songs in their original dynamics, but the drum parts would come from the audio system and he'd perform a duet with whole song through playing piano.
I could say tens of variations of this, believe me.
X's fandom is not full of child or stupid, so if Yoshiki say that he wants to quitting play the drum, literally everyone understand him and his feelings.
So, I believe that it's not about the playing drum or fear of injury.