This is a good move for Man United, as he can't be expected to replace De Gea. It's only right that Man United let him go if they want to make a change for the better, as Man United problem lies in their weak defense. Furthermore, even if he stays at Man United, the club won't necessarily use him, as Man United have already signed a new goalkeeper this summer, so competition for places is quite fierce.
Why sound like Onana was the only issue at Manchester United, that's not true because the cause it's a team problem involving both the management. It's normal for any player who leaves Manchester United to do better elsewhere, Onana leaving Manchester United is a redemption to himself and to his career. Manchester United has spoken less of Onana like he is the only issue, now he is out let's see how quickly they will evaluate the whole issue and sort out their crisis.
For sure we all know how bad Onana have been since joining Manchester United, he has made some terrible decisions in goal post that have cost Manchester United crucial points. But it just like you said onana is not the only problem in Manchester United but At least him gone is like picking out one bad egg out of many, it a way forward for Ruben Amorim and Manchester united.
I still have a big question about the Manchester United defense, in as much has onana is a bad goalie for them that defense don’t help matters too, most times they leave him exposed. Amorim needs to bring in more capable defenders if not this new goal Keeper might face the same fate.
I think Onana needs this move too, he was a good goal keeper at inter Miami, but life have been different for him since joining Manchester United maybe he might rediscover him foam when he leaves old trafford.