We had unique luck that we had Fergie. And because of that most of United fans lost sense for reality regarding managers. There are lots of myths there.
Myth 1. New manager must build his team.
No, manager must work with players that he has and make them better with addition of few players. If he thinks that he can't then he simply can refuse a job. In other leagues most of clubs work in a way that manager can't even decide what players will be bought. Not saying that he don't have any input but he can't ask for specific player by name. He says that he needs striker, defender and left back and that is it.
Myth 2. Manager needs time.
Yes he does but not few seasons and which is more important; modern football doesn't work like that. For managers is the same rule as for players; swim or sink.
Myth 3. You can't sack manager every season or two because you will lose stability.
Yes, you can. All big clubs do that. And not just big clubs, every club do it. When things don't work, they change a manager. Look at Bayern, Juve, Chelsea, Inter, Barca, Real...they even sacked managers who just won them titles.
Myth 4. If you give manager time, team will be perfect after few seasons.
That simply is not true. It is like you give playing time to average player and expect from him to be world star. And in manager's world it is even worse because many other things, except his coaching qualities, must click.
My opinion is (while i agree that is great to have one Simeone, Fergie or Wenger) that football is not working like that anymore. Managers (just like players) are expandable today and we should not be afraid to sack and hire managers. That doesn't effect on stability of the club.
Edit ( thanks to
@Skills and few other posters ):
Myth 5. Supporting manager = supporting the club
You support the club. And club comes first and second. Manager is just like every player, owner, coach, scout or CEO. Employee of the club. He can be criticised and praised just like everybody else.