The decision has been taken by the club. United replaced De Gea with Onana. whether the performance looks good or not for United. but we can see that De Gea is old and must be replaced for United who will play a match with a tighter level of competition. Onana was brought in to be a solution. Ten Hag has a job to do to make United's defense strong.
I personally think the decision to bring in Onana was quite right. Even though he has now conceded 7 times, there is more important work for Ten Hag to complete. not in their goalkeeper. some players who make mistakes too often may have to be removed from the squad.
It's true, De Gea is still one of the best goalkeepers Man United has ever had, even though he has shortcomings in terms of build up, De Gea still shows his class. ETH's goal in recruiting Onana is because he is a goalkeeper who is very good at build-up and that is something that De Gea doesn't have, so it is natural that ETH really needs Andre Onana's services this season. Indeed, in several matches Andre Onana was not able to perform optimally, in fact he conceded 7 goals in 4 matches.
However, as you said, it's not all Andre Onana's fault, in fact Man United's defense is very weak, especially when Lisandro Martínez was injured, it is very clear that Man United's defense must be improved if they want to compete in the EPL league.