To be honest, I’ve kinda given up xc riding anyway, and that has been replaced by trail running. Now, my mtbing is pretty exclusively DH, because I enjoy it. If I couldn’t do that any more, I’d struggle to find something to fill it’s place as nothing else I do gives me the same feeling. I do road ride as well, and would probably do it more, but it is a totally different feeling..
Ultimately, I’d cope. I have other interests I’d probably persue more: I might actually become good at playing the guitar, or photography.
Edit: I would still have to do something outdoorsy, though. Not sure what.