During the time I have watched figure skating the winners at the Olympics generally (with very few exceptions, e.g. Witt and Urmanov) left to turn pro, because they needed money. The same happened with those medalists that did not consider to succeed better at next Olympics, perhaps. Roughly, the winners left and the losers stayed. I would say that generally speaking the same still applies. Although today the matters are more easy, because some skaters can earn lots and lots of money as eligible skaters, so there is no need to turn pro. They try again and again for the elusive gold medal (or at least a medal) or the rest for a better placement to later be able to get a good contract in shows (if not in USA, at least in some European ones).
About longevity, I´m not that impressed just for a fact of skaters staying and competing in several Olympics. If a winner would stay to compete and win in two or three Olympics, that would impress me a lot. With the rest of skaters, my feeling is that they just stay for the purpose of doing better next time.