In the past 10 years, we have gone through so many useless players who would be fantastic ... if only everyone else around them was world class. Doesn't it get boring?
Onana isn't a good keeper for this level. It doesn't matter if everyone else around him is world class or pub league. A good player makes those around him look better, not worse!
It doesn't even require the players to be world class because you can see clubs like Brighton who have been doing it for years because their players are comfortable with the ball under pressure. It's just about characteristics.
De Gea lacked what is needed to play out from the back, Onana has those abilities but you still need the other players. This forum isn't exactly full of posts about how fantastic our defenders and midfielders are at playing that way for a reason.
If you think a good player should be able to make the players around him look better then you should also be able to accept that poor players (or poorly performing) can make a good one look worse.
While Onana hasn't been great overall and has been very poor in the CL, we have been all over the place defensively all season (and just poor as a team), that is always going to make it difficult for a keeper.