Firing Ten Haag is easy, but finding the right person to replace him is difficult.
Many good managers but not necessarily can succeed at this club, proven after Sir Alex Man United became a team that had difficulty achieving, they won several trophies but they were not favorites to be a title challenger.
The problems at Man United are becoming more complex, I am sure it will take time to fix this team.
I also think the same as you. Why Ten Hag has not been fired because it is difficult to find a replacement and it is not certain that the targeted coach wants to coach Man United, because we know that building a team takes a process but Man United fans are impatient with that process. How many times have Man United changed coaches but none of them have been able to change Man United for the better because they want instant change. Since Sir Alex retired Man United has been in chaos and the problems have become more complete, and it is true as you said they need time to fix this team.
Indeed, changing United to be better will take time. and the first step that club management must take is the right coach. remember, even if they get a new coach, United will not achieve big changes in a short time.
Many also criticize some of the players who are currently Ten Hag's choices. if the coach is replaced, of course, there will be changes in some players. this will be a long process. but if you want to see good progress, United must do it immediately.
Since the beginning many have predicted that Ten Hag would fail at Man United, why? Because if you look at the player transfers, it is more about nostalgia, it proves that this coach is not used to new players, wants to be in a comfort zone with tactics in a league different from the English league. In addition, Ten Hag seems unable to accept other people's opinions, it can be seen on the sidelines he disagrees with Ruud. Moreover, he does not accept input from players, who are clearly experienced in the English league field and some of them have played well with tactics and game plans that are different from the previous coach.
Ten Hag's attitude should be changed so that his performance improves, don't protect each other's egos. Because for now firing Ten Hag is also not the right solution, so Ten Hag must change and be willing to accept input from others.