I think that Mourinho was a victim of player power. Sure, he created a lot of his own problems with his demeanour but his record over the years speaks for itself. Come to that, whilst United are in a trough right now, his record there is none too shabby. He had a win rate of 58.33% against that of SAF with 59.67% ( Moyes 52.94, van Gaal 52.43 ) But it should be remembered that SAF managed in a different era when players and their agents wielded nothing like the power they do now. He also had the benefit of the 'Golden Generation' when the likes of Giggs, Beckham, Scholes and the Nevilles came through, players who took a huge pride in the club and forged a togetherness that doesn't exist in football these days. Add in the fact that United were THE club in those days, a club that all the top players wanted to join, and it was a far easier managerial job then than now. For instance, Man City, Chelsea, Liverpool and Spurs didn't offer the same threat as they do nowadays. Probably Wenger's Arsenal was the only team that did.
When SAF retired I think he saw the writing on the wall and the team he left Moyes didn't compare to what they once were. Football was changing. Citeh and Chelsea were beginning to flex their financial muscles and United's dominance was coming to an end. For me, United players, led by Pogba, downed tools because they wanted him sacked and any club knows it's easier to sack one man than replace half the team. And that's the problem with football today. The game is run by self-obsessed players and their voraciously greedy agents and is all the worse for it.