I think it is not even a coaching problem. Sir Alex Ferguson was one of the greatest coaches in the whole world and history but the dude managed to run that club for decades which gave him the chance to build a system.

Whenever Manchester United got a new manager they had to readjust to a new system and the players that didn't fit to that system even if they were good were cut from the team for some reason. That is why instead of looking for a coach that can give you success right now every team should be focused on getting a coach that will be in that team for a long time.

If they find even a semi-decent coach and let him run the team for 20 years they will have a big chance, they should not look for a Mourinho or Klupp or Pep but look for a person who is not as good yet will stay at the club for 20+ years, that is how success is built.