Frankly, it's all a bit media-hyped as to why Manchester United are supposedly so weak: the answer lies in there being no long term plan, no continuity. If you switch managers every other year and spend millions on yet another batch of players, you'll never get it right. Manchester United are a bit like their stadium, slowly becoming yesterday's news and in a slight state of decay. They don't deserve all the attention, especially if one puts it against the outstanding policy we see on display at Brighton and Villa: those are two very good examples on how to first build a vision, then build a team, then build results - in that particular order.
One shouldn't forget that the magic Erik ten Hag supposedly performed at Ajax, was because of groundwork done in the seven years prior to his arrival. Ajax was already a promising team (final Europa League) and the only thing he managed was for Ajax to do away with salary caps, making it possible to sign the likes of Dusan Tadic and Daley Blind and all of a sudden Ajax were on a roll. So, that whole first step is missing at Man United - no manager can make something decent out of that. An incidental good league or cup run maybe, but nothing structural.