Of course he was right, but some fans will never admit to it. In the end though, he became toxic.
With only the benefit of hindsight it is true his signings have largely failed, but when purchased, they were highly regarded, equals if not better than their peers at other clubs. Mourinho also had no say on the price, the blame lies elsewhere for that.
There's a problem at United and it goes well beyond any manager. Standards from top to bottom have dropped massively, things happen that would not have been acceptable less than a decade ago. Many fans look for someone to blame, often incapable of seeing beyond the manager, or a manager. Before Mourinho it was van Gaal, before van Gaal it was Moyes. It's a simplistic view lacking in any credibility once the same issues arise later.
United are a joke right now, there's no other way of putting it. There's a whole host of people partly responsible for this and they want you to blame Jose Mourinho, or the next manager, or the next. Some fans keep falling into this trap...