Erik Tan Hag staying at United means a lot, because it will give him one more season and he has something big to show. If he does well enough this season then yeah he is going to be considered a good manager, just one bad season (bad season with fa cup) will not be a reason for him to be considered bad, but if we are talking about a situation where it is clear that we are talking about another bad season?
Then in that case, it would not only be bad for him now, and he would be fired, but it would also mean that we are talking about something that will be quite terrible, we need to realize that. He could never be hired by any big club ever again if he does bad this season, his life would be lower and lower level of teams after this.
Personally, I also speculate about Ten Hag future fate if he fails to achieve his target with Man United again next season. This is an opportunity as well as a big challenge for his managerial career, his quality and strategy will be tested to the limit of his abilities, how much genius Ten Hag is in managing Bruno Fernandes and his friends. Ten Hag certainly realizes that he has to work harder to repay the trust of management and club officials, amidst the competition for trophies becoming more competitive next season. The pressure Ten Hag faces will be heavier, and it will be very difficult to realize his ambitions if Ten Hag does not completely control the dressing room.
It seems that Ten Hag has to change his approach to players so that harmonious relationships can be created within the team, giving full trust to players without favoritism. Without changing his approach, it will be very difficult for Ten Hag to encourage his players to reach their best level. Fortunately, last season the Man United dressing room was not too chaotic after Sancho departure. If Ten Hag fails again next season, of course only a few clubs will want his services, maybe Ten Hag could return to Ajax if he is kicked out of Old Trafford in the future.