I agree with you on this. It is true that they made a mistake by sending De Gea away and getting Onana, but it has been two seasons already and I do not see how De Gea would be getting back into United and helping after this age. He is 34-35 years old, and he is only getting older.
Onana was a mistake, and they should sell him as soon as possible, and they should find some other goalkeeper. The sad part about football world is that we do not have many under 30 year old great goalkeepers at the moment, not like we did back in the day. When I look at all the "expensive" goalkeepers from transfermarket, like the highest value, none of them are superstar names, none of them pop out. So United will just hope for the best when they replace Onana.
Onana is not underperforming; rather, he's performing to his best performance; I don't think Manchester United is letting him go that easy because he is not part of Manchester United's problem. He won't defend the goalpost and act like a defender at the same time. He will only act like the goalkeeper, that's it. Nothing above that.
The problem of Manchester United is from their players. If the right players are brought into the squad, Manchester United will have more match wins than loss matches.
For Manchester United, mistakes in defence have been the most effective throughout the matches. Manchester United probably have the most mistake-making defenders in the Premier League. I also don't believe that goalkeeper Onana has been at his best. He showed an extraordinary goalkeeping performance when he played in the Italian league, but maybe next year if the defenders are better, he can show us a different, good goalkeeping. I think he should leave the team now too.