Why have Manchester United consistently performed to the highest level for the last 20 years?
One word: evolution.
Sir Alex Ferguson never lets a team become stagnant. He tries to introduce new staff, tactics or players that will revitalize and invigorate a flat team.
United's young side may be on the verge of greatness over the next few season but it's up to Sir Alex to keep the squad ticking over with new additions that add competition for places.
One might look at the abundance of strikers that the Red Devils have now but a team should never be settled; it can become complacent. A new signing might spark a response from some other players.
One of the reasons why Chelsea are yet to depart from "Mourinho Mode" is due to the fact that it is still the same team. An experienced manager like Sir Alex would never let that happen.