Clubs usually look for managers with proven winning history to avoid risks and ensure good results. Ferguson huge success meant United trusted his judgment but if that trust led to decisions based on friendships over actual ability it certainly played big part in club decline.
It is clear that to succeed long term favoritism must be avoided in important choices like hiring manager. Even so your hope for Manchester United is valid. Many feel that with right choices and focus on merit club can definitely bounce back.
The era of coaches like Alex Ferguson or Arsene Wenger gone a long time ago and nowadays, big clubs globally not only in Premier League no longer have enough patience to wait for good performances from head coaches and their players. If results turn to be bad for several matches, a half of season or a season, the head coach will have very high risk of sack.
Manchester United have struggled to find a head coach who can manage their players well and achieve good results. Sometimes they came nearly to this target with Jose Mourinho but they did not have enough belief and patience on him. Other coaches have worse performances than Jose Mourinho while they were given more money, time and patience than Jose Mourinho and it's responsibility of Manchester United's owner and board members with their bad strategy.
Maybe Manchester United made the right move with Ten Hag, because he achieved good results with Ajax, if you compare Liverpool's decision to take Slott as coach instead of Klopp, everything is similar, the only difference is that Slott came when Liverpool were doing well, and Ten Hag came during a big crisis. And after he left and with the arrival of Amorim, it is obvious that things got even worse, so you need to take everything into account before drawing conclusions, and at Bayer he can do much better.
They must choose a right head coach and things will work well. A same strategy can work differently with different head coaches, and problems in Manchester United are their inconsistency with strategies, so they changed their strategies many times. With a good head coach like Jose Mourinho, they had bad strategy and distrust him. With bad head coaches like Solskajer and Ten Hag, they gave them too much time and trust.