I will be ranted at for this by Red Devil and sorry Man Utd fans this is my opinion but I think Manchester United maybe beginning a slippery slope what with Ferguson leaving soon and sale of key players. I think they will stay on the slope but they will need to make good signings.
I agree with you, every football team has its cycle and perhaps United will soon see the darker side of theirs. Strange that this should be said on the back of a near treble or even quintuple, but...
It's not everyday that a huge club like United gets outmonied by another club. Also, obviously they have the debt concerns to deal with, some crappy owners, and the potential departure of their greatest ever manager. They do have youngsters coming up, but in the Premier League in particular, it's not always the way to go. And they may just start to look out of depth soon. What they do in the transfer market, as Viva said, will be crucial. One thing I'm pretty sure of is that United won't lose their popularity anytime soon, and that is obviously going to help them through tougher times.