I just think the main problem United have at the moment now is no identity. By that I mean Barca/Real/Bayern top clubs have a "way" of playing that seems entirely seperated from the current manager, they have an overall philosophy top down which means even with a blip they move in the right direction. Feels like ManU have no top level leadership and then their direction is changed by each successive manager, who add their own signings then they end up with what they have now. They'll always be thereabouts but I can't really see them competing when it comes to the crunch. Look at the signings, so many number 10 type players in the team, no real width, no real decent DM, still questionable CB's this is after spunking (this is a rough guess) probably 350-500 million in the last 5 years? .
Personally think its unfair to compare Utd to any of those to start with. Barca and Bayern dominate their leagues year in year out without rarely having to get out of 2nd gear. When a manager swap happens, they already have a very good if not exceptional team to work with from day 1 so dont need to change anything , and they might bring in a promising youngster or change one or two senior players but nothing that alters the team majorly. Even though Real dont dominate quite as much, they still have a relatively stable 1st team that managers dont need to change that much to still perform well in 9 out of 10 matches.
Utd havent had even a very good team (ie functioning as good as they should be) for ......4 seasons? Fergie patched over a few things in his final season but it really wasnt a great team, Moyes didnt do anything at all and Giggs didnt really get a chance to do anything. LvG was a fish out of water most of the time.
Taking off-field considerations out of it for a moment, Utd have available players that would make a pretty decent (once they were used to it) 4- 3- 3 team and it would get the most out of the majority if not all of them.
Darmian , Smalling, Bailly, Shaw, Carrick, Herrera (or Schneiderlin), Pogba, Martial, Ibra, Mkhitaryan would be a really good line up - solid at the back but with creativity and speed up front with a lethal striker (even if he has missed some relatively easy shots , for him, recently)
Henry and a few other commentators seem to think Pogba excels at No10 and slightly less as an AM, either way he is a whole lot better than playing where he is currently and not always catching the runners behind like his current position requires.
Whether the legal behind the scenes issues doesnt allow for that I dont know, but its looking like Rooney is simply undroppable - and with his form currently that would ***** up any of the teams you mentioned.
Utd may well have "wasted" a lot of money in that time frame, but with no consistant base to work from , its not surprising different managers want to change different aspects to start with.
Premiership teams just cant "carry" 2 or 3 players any more due to the competivness of the league. Utd are currently carrying at least three (Valencia, Fellaini and Rooney) with an out of form Martial and an inexperienced Bailly. Martial carried Utd through multiple games last season nearly single-handedly, its unfortunate he has lost his form but Im sure he will find it again soon.