I wouldn't call managers like Mourinho or Van Gaal bad, these are the best talents you can get for the money in the current market. The bigger problem is the eagerness of the fans and supporters. They want instant result. It took 3 years for Klopp to finally win a trophy at Liverpool. But Man Utd was so accustomed to win titles in Ferguson's era that anything less than that is a bad season for them. Solskjær was doing great but then Ronaldo came and he had to change his plans and it backfired. Now, Tan Hag who is one of the best European managers but already facing backlash after a single loss. Only if fans can remain supportive to him for a season or two like Barca doing with Xavi, he can turn the wheel of United's fortune.
Van Gaal was past his best days when Man United gave him the call and got him in the club. I think the biggest mistake was hiring the genius Moyes. It would've been different if they went straight to Mourinho. He wasn't deteriorating yet at the time and he has always expressed his desire to manage United after Ferguson. What could have been...
There were also some words swirling around at the time that Ferguson went to Barcelona to give Guardiola a visit during the latter's sabbatical and to ask him to be the next United manager. He went to Bayern but I think Pep has always had his eyes set on City after Ferran Soriano and Txiki went there.