I understand your point mate, Ajax is not as big as Manchester United which might the reason why he could succeed in Ajax and couldn't in United. Some coaches cannot face the pressure when coaching a big club because they will be worried and confused due to critics from the fans. Those coach are known as average coaches. A professional coach will succeed in a big club despite whatever challenges that they are passing through. Look at Xabi Alonso who brought Leverkusen out from the darkness.
Ten Hag did not succeed at Man United because the club is rotten. How many managers have Manchester United hired since 2013, and how many have done well? In that time, the only manager you can make a case for is Mourinho, and that is because he won the Europa League, but if you want to look at it properly, even though he did not succeed at Manchester United, will you call him an average coach?
Man United need a world-class coach like Pep, Conte, Klopp, or Flick, then give the manager the freedom to control his team. He decides who they sign and who gets sold. The manager should be allowed total control of his team. This is the only way they can stand a chance of making it back.
Sadly, this is not going to happen because they won't hire a world-class coach, nor will they give him the freedom to run his team.