This isn't foo far-fetched. I think he knows his body is beaten up, and he has a sort of resigned look on his face all the time. He doesn't even appear too fussed when he loses. Let's face it - he's done it all, won every slam. What has he got left to prove? I forecast a retirement and a life of fishing in the Mallorcan sunshine within 18 months.
I was going to say he could pursue his golfing activities, but that can be hard on the knees too if played too often, and it's his knees that surely must be his main problem.
I know I've already said this on another thread, but about a year ago I read an article on his website in which the consultant who treated his knee problems said that the tendons had actually disintegrated, that he had patched them up, but had warned Nadal that if he didn't change his style of play and also played fewer tournaments they would only disintegrate again, and that if this happened little if anything could be done about it second time round. It certainly doesn't seem that Nadal has heeded this advice. He has a very strong competitive streak and I'm prepared to believe that he'll go on until it's proved beyond a shadow of a doubt that he simply can't.
Very interesting that Nadal is elected to spend more time practising in order to get his game up to its old level before Paris instead of playing an extra tournament. This smacks a little of desperation to me.