I'm still not clear in my own mind why it has failed with Amorim and therefore what it is that's required of a new coach to be a success.
a) Did Amorim fail because he wasn't adequately backed by the board?
Well they spent a ton of money again but was it best spent. Did Amorim want two tens when he had Mount who he seems to prefer if fit, Amad and Fernandes who is one of the worlds best. I never got the signing of Mbeumo after we got Cunha. Certainly not ahead of a central midfielder. I can't believe wanted to forego that position when it was clear he didn't really fancy Casemiro at first, Mainoo struggled to persuade him and he called out Ugarte towards the end of last season for not being the player he had at Sporting.
b) Was the 3-4-3 system inherently not suited to the EPL or is it too rigid in the player profiles required to play it successfully?
The easy answer here is to say that its both and Amorim was naive to think that what was successful in Portugal could be equally so in England. However, the board knew this when they hired him. They would also have known that the United squad did not profile good matches for this system. They bought him Dorgu - a kid from another league, asked him to convert Amad and used Dalot to fill in wherever there was a need. This resulted in no threat going forward whatever side Amorim or Mazraoui played.
c) Was Amorim just out of his depth?
To an extent yes. The intensity of the English league and the press surrounding Manchester United would have been a shock to the system. As a naturally engaging guy he gave a lot of himself. Arguably too much but he also got no slack for the fact that second language english speakers will always make mistakes in communication. However he showed toughness in sorting the bomb squad which few fans would argue against. He built a stronger team spirit for sure. I think the players liked him. They worked harder, they pressed better and this season didn't lose games that they did under ten hag and last season.
d) Is the job impossible?
10 managers in 12 years suggests so and ultimately this comes back to the Glazers dry raping of the club.The constraints placed on Ineos mean that any significant rebuild was always going to take longer than is acceptable to stakeholders. On reflection the ten Hag and Solskjaer periods really set us back in terms of squad development. The debacles with Ronaldo, Sancho, Antony, Greenwood, Onana, Hojlund set the club back massively. We have until this season bought really poorly. van der Beek, Casemiro, Eriksen, Mount, Zirkzee, Malacia, James. All symptoms of having no strategy and arguably too much managerial input or commercially motivated deals.
There is no doubt Amorim under-performed. The stats don't lie but the rebuild had to be wholesale. The dressing room was toxic. Players had too much power and he has had to deal with all of that. I think he made progress but I don't believe he would have picked Maguire, Shaw, Dalot, Casemiro to be so integral to his squad. These players are not at the level of a top team. We are asking a lot of Dorgu, Mainoo, Heaven, Amad and Yoro talented as they are.
So in terms of what is required.
Strong will, media savvy, able to manage upward, credibility with the squad, tactically progressive and a committed to playing the United way (whatever that is) and presumably some sort of winning track record in a top 3 European League? Importantly they also have to want the job which I don't think is a given.
Personally I think the board should have signed him at least one midfielder instead of Mbeumo and allowed Amorim the season. Otherwise they are quitting when a job is half done. As
@sigma commented - the goal scoring has improved but the defending as a team is a problem. I don't blame the back three. Shaw aside I think they have been decent but we have lost a lot of goals through not tracking runners from midfield. DeLigt was out best player until he got injured.
If it were up to me and they are only going for an interim, I'd leave Fletcher in charge until June. I think he will be ok and fits most of the criteria aside from the winning experience. He's a football geek, United through and through but also knows what its like to play for West Brom or Stoke. Pulis's testimony about him is unbelievable. I don't want to go back to Ole and offering Carrick it for 3 months doesn't feel right. I don't think he's quite the right personality.
Fletcher is a low risk appointment. He's humble and not a huge ego, if he fails then it won't be any worse than Amorim and if he does well he'll still likely be happy to slot back into his u18 role.