My new theory why Alex Ferguson retired at Manchester Untied?

why did he leave?

My new theory of why Alex Ferguson retired from Premier League winners, Manchester Untied. Alex Ferguson always made it clear that he wanted to beat Liverpool in the League titles and in Europe, but why did he quit?
My Idea is that the Manchester Untied team is very old in players such as: Robin Van Persie, Rio Ferdinand, Evra, Carrick and many more big players and he must of thought a serious investment in the team has to be made to get young talent in, and the owners couldn't do that. Alex Ferguson wanted to finish at the top as many managers would like to do and left all of the work for David Moyes. Watching Manchester Untied their is no class from their players. They are scrapping out wins and draws. In my opinion they need at least £150 million to bring in young talents such as: El Shaarawy,Draxler, Baines, Papadopoulos, Will Hughes and Benzma from Real Madrid.