I think you are ignoring all those silly mistakes he do make also. Everyone knows Manchester United defense is wack but we also know of the fact that Onana can do better and prevent hid silly mistakes and make clinical saves.
Onana should more likely become a second choice and maybe with this he might actually better and being sold a fee that would make them not to be in a bigger lost.
I wouldn’t seriously accept some of your opinions here, even though I think Onana bashing is sometimes really deserved because he usually makes mistakes that I can say are silly. As a goalkeeper I can say that there are situations where you need to be at your best performance even when the team is playing badly so that you can keep them in the game, a clear example was Allison in the champions league first leg against PSG he outrightly kept the club in the game until they managed to win it.
Has Onana ever done this I can say yes few times he has kept Manchester United in a game either when they were losing or when they were just a goal down, but the problem has always been consistency, Onana isn’t consistent in my opinion and this has let the club down in couple of times even at winning positions.
My opinion is, he is good at his feet and on his day can give you results but he clearly needs competition something which Bayindar isn’t offering at the moment. so for me it is actually getting another great goalkeeper like Maignan of AC Milan to provide competition for him and I think he will sit up or be sold the following season