BR - Alex Ferguson is a legend. There is no question, no doubt, no valid argument against such a statement, and if there is, I'm yet to hear it. That said, he cannot go on forever. A fact he himself acknowledges at various press conferences and post-match interviews with increasing frequency. However, what happens when he does eventually vacate the most prestigious (and daunting) job in World football?
As for replacements, I would much prefer someone like David Moyes, because he has proven himself in England year after year after year with no money. Huge respect for what he has done with Everton and the time he has lasted- nearly 11 years now, third in all England behind only Wenger and Ferguson himself.
United need Moyes to come in who is still not even 50 years old yet, and stay at United for 20 years. He could continue the stability and not suddenly want away like Mourinho or Guardiola when things don't quite go perfectly....