Van Gaal and Mourinho are definitly above avarage by international standards. Moyes beyond avarage by Brittish standards. But as said - neither top by their time managing United, but above avarage. Solskjaer was not.
There is, or has been, a lot things preventing a top manager getting us to play like a top team. At the moment United are restructuring and hopefully they will get better at keeping decisions on football and business apart. Both United and Ten Haag should have observed and learned from the clubs past misstakes and Ten Haag would probably not have taken the job if he wasnt assured otherwise (and the United structure is probably also tired of their own failings).
The United hierarchy has prioritized markatable players, top 4 finnishes and revenues and have been satisfied as long as the business is running. Chelsea and City do also have wealthy owners - but they are in it for both geopolitical reasons and prestige/glory. They want to win. The Liverpool situation is closer to Uniteds but they do not lean on the same kind of star quality and international brand as United - they needed a succesful team to make the business grow. This, or they would have had to go for the smaller Arsenal model.
It is not necesserly true that United managers has been able to get the players they want and implement their style. Player power has been huge and the incompetence of Woodward has been for all too see. Trying to sign the likes of Bale, Griezmann, Fabregas, Ramos etc. and ending up with Fellaini, Mata or an overpriced Martial. When United under Woodward did manage to suceed with bringing in a star name like Di Maria, Pogba, Sanchez or Ronaldo it was always without a plan for how the particular player would fit into an idea of play. The name came first and the managers had to deal with what they got. Key areas as center back and defensive midfield was not solved when needed or asked for. Three out of four managers have been visably frustrated with how things worked at the club with Solskjear being the exception (who would do anything to stay at his dream job). He also suited United best comercially since he was able to bring back culture values and "a team to like" with the Ferguson nostalghia and mix of Brittish players, young players and players "that gets you of the seat". The board was really reluctant to let him go eventough terrible results since they where satisfied with the vibe and their assets happy/player power intact.
The board and their lack of football ambition and sheer incomptence (Woodward getting played on the market again and again) has for sure been the biggest problem. Also responsible for appointing such a random mix of different managers and undermining their authority. Hopefullt things will change a bit now, but its not for sure. Everybody are positive at the moment and Ten Haag seems strong but if shit hits the fan and business people take business decisions again it might be trickier than what he hoped for. The earlier managers also bought into that things could and would change. They didnt.
Is the structure the biggest issue? Is it really? It's part of it, but there's nothing in it preventing a top manager getting us to play like a top team. They have time and freedom to bring in their own staff and coach their style. They have the finances to bring in players they want. We have played like a disjointed team with no style, because we've had managers who didn't know how to train a style befitting of a top team. At least not one that can match up with other top clubs. And is that a surprise that Ole or Mourinho couldn't match up with Pep or Klopp on similar resources? You say above average... Ole's previous experience is getting relegated with Cardiff. If a manager who was relegation tier can come in and finish 3rd and 2nd with multiple semi finals and a cup final, is it really an impossible structure to work in?
Sometimes, it is really that simple. The structure hasn't been the best, but the biggest problem with the structure has been the terrible appointments and a lack of cohesion in recruitment between managers. The managers have controlled the plans essentially, so the new guy would change from the old guy. Which isn't an issue if you just have a good manager in charge who doesn't get sacked.
