The main problem with Man United after Ferguson wnet out, was and is the bad transfer markets they have, they spend so much for players, and all the teams what trade with they exige so much money knowing Man Utd pays ever.
For bad players or good, they pay a minimum of 30/40 millions for nosense players.
While that might be true, they've got fantastic players. They actually had world class players in almost every position. Pogba despite the criticism he gets, was exceptional at what he does, but it seems he could never get going in a United shirt, and was never given the role he needed, regardless who was manager.
They still have fantastic players, but obviously something has seriously gone wrong at the club level, as we've seen multiple managers come, and go now. Players actively being vocal against the club, doesn't help morale, and while I always thought Ronaldo was a bad idea to bring back, he isn't helping the situation at all right now.
You're spot on with every points made, especially on the terms that most of the players those respective managers got were fantastic ones, but once there's a change in coach, there comes a new preference of players and some of these players gets played out of their preferred role which the initial manager got them for. Di Maria was a supposedly a flop at Man United but went on the to be back at his best at PSG, Pogba was also mediocre at Man Utd but that's what you'd expect when players have to adjust to a system entirely different after almost every 6 months or calendar year, I really thought when Mourinho joined the club, that he was going to be at least a 5-6 years deal at the minimal, and he was the best manager Man Utd had since Ferguson, and won them major trophies and that's because he stayed longer than other managers lasted at the club, I'm not a fan of his football tactics and ideology to say the least, but I think If Mourinho had remained at Man Utd till now, they'd be way better off right now.