1. Having no proper thought out/reasoned 'succession planning' process in place
I'd agree to an extent, but allow me a counter-argument. I am assuming you mean by "succession-planning" you're implying the manager.
I'm not saying you and others are wrong, but I would like to posit an alternative evaluation and my perspective and why I disagree with so many people citing the Manager decisions as the biggest issues.
First of all, I'd like to ask: which teams have had successive periods of success that coincide with something resembling "succession planning" for managers?
1) Real Madrid? I'm honestly not sure how Mourinho -> Ancelotti -> Benitez -> Zidane -> Lopetegui -> Salaro -> Zidane -> Ancelotti in any way, shape or form constitutes succession planning. They racked up more Champions Leagues in this period than just about anyone. Any United fan would give a nut for that success. Is that proper succession-planning?
2) Chelsea? Their manager appointments seem to have been haphazard at a minimum. But they had serious run of success where it looked like the manager didn't matter. This is also the Chelsea that shipped out Salah, Palmer and De Bruyne. Marc Guehi, too. What would a Chelsea with those 4 players look like today?
3) Brighton - Ok, this is one we can accept but the expectations aren't anywhere near United's.
4) City - If succession planning meant getting in a structure for Pep, than yep. That was success. But Pep hasn't moved on. So we don't really know what succession beyond Pep would look like.
5) Shitepool - Ok, Klopp to Slot looks great. Give them that. But before Klopp, was it multi-year succession plan? I doubt it.
My point is: In most cases, it's actually about having a good squad with good players, the managers have been almost interchangeable - success comes with the players available.
This is why I don't think United's biggest mistakes are purely down to manager decisions - rather that our manager decisions compounded issues.
Let's go back and look at the managers:
1) Moyes: I'd never be bold be bold enough to question SAF, but we have to be honest and even go back and look at posts around this time - especially before RvP and that last title. The squad was aging, we weren't happy with (quite a lot) of the players.
2) LvG: He was just plain odd. He signed Depay, who showed up in a fur coat - and got played in a different position every match for like 5 matches and then disappeared. He signed a lot Dutch players, and players who he saw in the World Cup when he was with the Netherlands - Rojo, etc.
3) Mourinho: He was a contrarian who always seemed unhappy. He wanted former players, established players, basically anyone but what he had. At the end, the narrative is that United didn't back him in the market - but you have to remember who he wanted to sign at the end of his reign. I'll let others cover this, but it was horrifying.
4) Ole: It was all British, all the time - Brexit FC. Until Ronaldo Mk.2 destroyed everything.
5) EtH: Oh, he was backed. One accusation you could never level at the club is that "we didn't back the manager." It looks like he bought fecking shite.
So, if I had any thoughts now, it's that "maybe we backed the managers too much". This leads to that, "well if we had a proper DOF in charge we'd be successful."
Well, who were these DOFs at Chelsea, Real Madrid, Chelsea, Brighton, City and L'Pool? I mean we finally have City's Berrada now... maybe that''ll be success?
My posit: Success simply comes from having good players. Not even the managers, although they help. Not the owners, although they help. Not "long-term planning", even though that helps. We just haven't had good players.