I cant speak much about La Liga but in Italy there are two clubs who produce/refine most of the managers. These clubs are Milan (Maldini, Trapattoni, Capello, Ancelotti, Donadoni etc) and Juventus (Zoff, Deschamps, Zidane, Conte etc). Young promising managers tend to love going there because they give you structure and they teach you the ropes. People like Allegri, Lippi and Sacchi have a lot to thank Juventus/Milan for and their football is copy and paste Juventus/Milan style.There are clubs who are capable of producing young managers and unfortunately we're not one of them. Throughout our history we weren't able to produce 1 top quality manager and you only have to spend 10 minutes listening to the likes of Gary Neville, Lou Macari, Ryan Giggs, Philip Neville or Robbie Savage to understand why. While other clubs build a hierarchy of roles with football directors supervising every aspect of the game and giving advice/instructs what the manager should do, United expect the manager to handle everything. The only way that a Man United manager can succeed is to be an absolute genius. No wonder why players stand at awed at him and no wonder why no frigging rookie can hope of ever doing it. Moyes had 10 years experience at Everton and even him looked out of depth at United.
The only way United can produce top managers is for us to rethink the whole system up. We need top football directors who are experts in all aspects of the game and who are able to advise and when necessary step in and stop the manager from doing mistakes. That happens all the time at Juventus. Spoiler alert it occasionally pisses off managers enough to leave. Just ask Lippi and Conte. Till then lets be content of being able to produce good foot soldiers with a lot of heart but little managerial brains. People like the Nevilles, Giggs and Savage.