De Gea's saves are more reliable than Onana's, the Red Devils need a good goalkeeper like De Gea. Onana has been tried and it is not possible to say that De Gea is good either. I think De Gea's departure was a mistake in the first place, he left the team because he did not fit Eric Tan Hag's game plan, this goalkeeper was always good during his time at United, it is interesting how he did not fit Eric Tan Hag at that time.
I do not see De Gea coming back to Manchester United as very likely.
Honestly, I don't think we should be comparing De Gea and Onana. The two of them never had the same time of play and the circumstances were different. When De Gea came to Manchester United, the club was one of the best that if they don't finish first, they will finish second place in many leagues they are and De Gea didn't made an appearance mist often until later and mind you the club position was very rich with plenty of players that were ready for the club.
However, Onana case is different because he came when ETH was trying and struggling to make good place for the club and everything he did failed and the pressure is there on every player. Right now, Onana is facing so much that it's very difficult for him to do much for the club, he is afraid of making mistakes so he doesn't get bash but the same fear is making him perform very poor, this was the same thing ETH saw that he never fired him because his past performance, he did well for Inter.