I dislike the notion of "going out on top", "if you're not first you're last" mentality regarding Phelps and Bolt. If they genuinely don't love their respective sports anymore and don't want to compete, more power to them, but people saying 'don't be selfish, let new blood come in, don't tarnish your legacy', to be a sportsman goes beyond that.
This is what Roger Federer said to those who want him to retire so he doesn't keep losing and ruining his once near perfect slam and open era record.
He plays because he loves the sport. We still see spectacular tennis because of it even as Novak and Andy have taken over. It is what they have done since children. To play just to win and then stop just so you don't lose a perfect record or becauae you won't be the best, makes you forget why you did it when you weren't the best. Bolt can still win in Tokyo, as can Phelps. There is still gas in the tank.