JP credits his condition to just pure luck, per some interviews I read. He says other guys who use good technique and take care of themselves just as well as he does get those injuries and conditions from playing guitar and he just doesn't. Kind of scary to think that all kinds of world class careers just hang in the balance between genes and the way someone learned to move their fingers on the instrument when they were 5 or 8 or 10. And in a way that none could be the wiser until he actually gets that injury or that condition and a doctor looks at him and says "well in hindsight it was that repetitive movement that got your ass." Not that people would take away JP's world class career if he could suddenly play 10% less of his own stuff but he would cease to be that guy he is now and become someone else.
I think your post is going to be another thread, haha! I was thinking about this the other day in an un-related context and it struck me that no doctor worth his salt would just say "eh, you're this because of dumb luck!" We - humans - LOVE to see patterns and love to be able to attribute cause and effect, even when it doesn't actually exist. I would even argue, our survival as a species depends on it.
The reality is, too many factors are involved, and too many variables for us to reliably track. Does his weight lifting help or hurt? Might help, but one side effect of big bulk is limited flexibility, a negative when you're playing an instrument, so who knows? Does his eating meat help or hurt? Whatever drinking he does (I've never heard any rumblings of a problem but he has a liquor brand so I'm guessing not a tee-totaler), does that help or hurt?
Steve Morse - hero and friend to John - is a great example; the man plays obsessively, and he's spoken candidly of his need to adjust his style to account for physical limitations (he has written about pain in his right wrist, and now wears a significant brace on his arm when he plays, and sometimes looks to be in pain while playing; I noted this during a Dregs show where I was in the front row, as well as during the Flying Colors show I saw in NYC.).
I think luck plays a big part, but that narrative doesn't really cut the mustard in a society that is predicated on "anyone can do it with enough hard work!"