The fact is that everybody around a college basketball game - the coaches, the announcers, even the referees at a lower level - calculates when the game is really over. They calculate it with intuition and guesswork.
We need new athletes all the time because we need new games every day - fudging just a little on the definition of the word 'need.' We like to have new games every day, and, if we are to have a constant and endless flow of games, we need a constant flow of athletes.