Every "top" team goes through a period where they can't dominate anymore. They're past their peak and into decline, age has caught up and other factors.
Fergie dominated for soo long because he was able to freshen his side, correct me if I'm wrong but during his 20 years I'm sure he created 3 sides.
It is all about getting the right players in due time.
I can't remember though how Fergie did it, did he spend over the odds during those times??
He did have tough competition in Arsenal and then the lottery winners Chelsea of those days.
Maybe someone here has a better insight into how he was able to keep creating a side that was challenging.