Really ? History says that wasn't the case at all though it's an urban legend that perpetuates.
I don't remember Ferguson selling on his best players unless he was absolutely forced to by circumstances; Ronaldo, Beckham and Keane after a falling out spring to mind, though even he was getting on. His mainstays (Giggs, the Nevilles, Ferdinand, Cantona - retired, Rooney, Robson, Bruce, Schmeichel were there until pensionable and others (Vidic, Solskjaer and the like) were sold when way past their best. It's a myth that Ferguson rotated his best players from a plan and not necessity.