I think Manchester United lost that champion mentality where they would want to win everything and I blame Ten Hag for that. He already had 1 and a half seasons with this squad and he couldn't do much with it. As @Ahli38 said, the team morale is very down. Nobody seems happy in that squad. Manchester United has nothing going for them currently. They are out of the Champions League, and currently at the 7th position in the premier league.
I think the cure for the United Squad is to sack Ten Hag and have a new manager who can start rebuilding the squad starting from now. If he starts at the next season, half the season will be gone just rebuilding the squad. Currently this season the team has nothing to lose. A new manager would come and rebuild the squad without any pressure. And Slowly they can regain their champion mentality.
I have the same ideas as you, but I don't know who can bring this team back to the old days. After Alex Fergsun, many managers came, but none of them succeeded. Do you have anyone in mind? Which manager do you think can take Manchester United back to the old days? Whoever comes to this team will be under a lot of pressure. I think it's not easy to be the manager of Manchester United.