Both things can be right at the same time.
Moving on from DDG was probably the correct thing to do. Signing Onana is looking like its going to have been the incorrect thing to do.
Hes still got time to improve. Definitely needs some time out of the team though.
But it actually wasn't and no mythology will ever make it so.
Despite the undeniable mistakes De Gea made last season, our defensive record last season was outstanding. It was our attacking record last season that was horrendous. In an epic thread over the summer the argument was made that getting rid of De Gea and replacing with him with Onana would improve our attacking record. But that argument is as daft as arguing that bringing in a top CF would improve our defensive record.
Keepers exist primarily to keep clean sheets, or to put it more generously, keep the opponent's goal tally as low as possible. We can all point to the Benrahma goal or the Brentford goal early in the reason as De Gea mistakes in the PL -- don't worry, we'll get to the EL in a moment -- but over the totality of the PL season our defensive record was fantastic, even acknowledging the pasting we took at Anfield. De Gea was an important part of that.
As for the EL, every single one of us is aware of De Gea's calamitous mistakes at Sevilla. But few seem to remember the mistakes our defenders made in the first leg that threw away the tie as every one of us knew that once we botched it v Sevilla at home that our goose was cooked at Sevilla, which began with a ball from De Gea that Maguire should have been able to easily manage but didn't, after which it all fell apart. But the more important point is that we didn't need the EL and many of us dismiss the EL as a joke competition. By then we already had CL qualification in hand -- not because our attack was so fantastic, but because our defense was so fantastic. De Gea, despite his mistakes a Brentford and West Ham, played a critical role in United qualifying for the CL. But what did we do? We got rid of him and brought in a keeper who has already made more mistakes in 8 games than De Gea had all season.
Genius isn't exactly the first word that comes to mind when describing United management.