I think it's a little too late for them though and If I was DDG, I would never entertain any nonsense of returning to Manchester United. From the past observation in Football, good goalkeeper usually get better with age since they have so much experience in-between the goal posts.
Let them just concentrate on Onana and not fumble him too. Maybe he could also turn out to be much better with age.
I don't think De Gea will return to the United as he's comfortably playing for Fiorentina. However, who knows he can change his mind. It may happen as long as United offer a little increased salary for him.
As for that observation, it makes sense considering so many goal keepers, which are 27 - 35 years old range still perform very well. Even some are just getting even better.
Lastly about Onana, they can't only concentrate to him. He obviously lacks of competition. I'd rather try to bring a quality goalie (not De Gea) to add more competition. The bigger pressure, the better for the player.