Michael Phelps has been questioned numerous times over the past few days regarding his stated retirement. His responses have been clear and concise, this was his last Olympics.
He will finish as the most decorated Olympian to date, a record that most certainly will stand for many years. This is what champions do, except for uncontrolled circumstances they finish on their terms.
If, on the other hand, Phelps were to continue swimming what would be his goal? In his interview with Bob Costas, he stated that he has reached all of his career swimming goals. He has also become the Michael Jordan of the swimming world. If Michael Phelps were to seriously consider a 4th Olympics, it would have to be for something greater than just the love of competition. For Phelps, the only result that matters is gold.
Champions at the level of Phelps love competing and winning. Competition for the sake of competing is not worth the time and effort. The idea of competition for the sake of competing is an anachronism from an old Olympic Ideal that never really made sense. However, if Phelps were to compete again in 4 years every aspiring Olympian will have to double their efforts if they hope to win.