The elite don't compare themselves against 7 billion people. When you've been the best of the best (and possibly the best ever, like Roger Federer has) you ain't gonna hang around too long as you slide in the rankings (unless you're in serious financial trouble.)
I think IF a dude retires on top, he's a legend.... but those days seem to be gone.
Michael Jordan left the NBA on top... TWICE! Doesn't happen like that much anymore. We saw it in BBing when the dynasties were allowed... Arnold, Haney, Yates, Coleman... Not like that anymore, in most sports.