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.
Personally disagree with many opinions that blame Andre Onana amidst Manchester United's very poor performance and the decision to release David De Gea, inconsistent of Manchester United performance this season actually not all bad performed of Andre Onana but how weakness United's defense easily conceded goals every match. Before joining Manchester United, Andre Onana success winning Serie A trophy and bring Inter Milan reach UEFA Champion League final round at season 2022/23 although defeated by Manchester City at the final.
Andre Onana is not fatal mistake for Manchester United and don't have to sell him for next season but need improving for signing new holding midfielder and defender position, its most crucial players needed by Manchester United if want improve their performance and reach the higher standings position after this season likely will finish outside from UEFA Champion League or UEFA Europe League standings spot.