No club will (or should) back a manager over the players if that manager isn't good enough. Letting a failing manager gut the dressing room is just making it harder to move forward.
There were already big question marks being raised over Mourinho in the second half of his second season. Our playstyle was poor, most of his signings had been expensive failures, his comments and quotes were already starting to try to shift blame to everyone but himself, and it already appeared that the dressing room was starting to fracture (something that has happened at every single club that Mourinho has ever been at for longer than two seasons). And in hindsight, the players that he wanted to sign at the point where he 'wasn't backed' have all (with the possibly exception of Perisic) gone on to prove they would have been poor signings, just like most of the players that he did actually sign. The fact that Maguire is public enemy #1 for much of our fanbase yet from memory is clearly in the top two best (alongside Perisic) of those signings that he wasn't 'allowed' to make says it all.
Then we followed him with Solskjaer. Short-term he was a good appointment. Turned that poisonous atmosphere that Jose had left us and got us going again, helping our young team enjoy their football again and encouraging them to play a more attacking style. However we needed to either leave it as just a short-term appointment and bring somebody else in fulltime, or at least surround him with established quality coaches. We did neither, and although we did well in patches there was never any real structure to our play or development of our players. Most of his signings were also poor. As such, why would we continue backing him when things absolutely go to shit?
Ultimately, we had a dressing room filled with young and talented players. Between Mourinho and Ole, we basically wasted six years with no real coaching or system to help those players develop. Now we're looking at those players saying that they haven't developed and Mourinho was right to want to get rid of them, ignoring that he and Ole (and the coaching team they surround themselves with) are a big reason for that. Not the only reason, but there's not a doubt that they are a contributing factor. If we'd had proper modern style and coaching system throughout the last six years I fully expect at least some of these players that we consider failures to have done much better. We're not talking one or two years here; it's six years which is a huge chunk of a players career.
Managers should get the amount of power that they ultimately deserve. There seems to be a myth that other managers are getting huge backing while we're the only club favouring the players. In reality do you think the likes or Real, Barca, City, Chelsea, Bayern, etc would have backed Mourinho or Ole as much as we did if they'd had identical performances for them? Of course not. They would have had even less backing and those clubs would have just got rid of them and bought in the next manager. In the time that we gave those two they would probably have gone through three or four managers in their quest to succeed.
Ten Hag should get a similar level of backing as what we showed Mourinho and Ole in their first year or two. It's then up to him to then prove he deserves that level of power going forward. The more positives he brings, the more power he should have to wield.