I believe one of the biggest problems for the club and probably for quite a lot of fans, in the last ten years, is that they got fixated on a manager who was going to give them success for another 20 years.
Fergie, as great as he was, also put the club and supporters, between a rock and hard place. His success was so incredible and regular, that it's not surprising that any manager or fan thereafter, was caught in a loop of believing they should keep the momentum going for another decade.
The same thing happened with Liverpool, after their dominance in the 70s and 80s, virtually everyone who was there watching that, still believed they'd win titles in the 90s and 2000's. But it actually took decades for them to win and bounce back under Klopp.
Football has moved on, more clubs have more money and can compete. The 'top four' has now become the 'top six', it won't be long before that becomes 'top eight'. Gone are the days where a club dominates for 20 years now (City's cheating aside).
For me, Uniteds biggest failing is still expecting a manager to come in and produce a team that's going to 'do a Fergie'.
I think the club and fans reality needs to reset and they just need to be happy with a manager that might give you 5 or 6 potential good years, not 20.
I'm not defending ETH here by any stretch, personally I don't think he's up to even 5 years of title hunting.