I think he will.
It will be an epic comeback if De Gea return to Manchester United, especially if he can prove if Manchester United was wrong by making many saves. A goalkeeper can still able to play in top league up to 40 years old e.g. Buffon, Neuer, and Casillas.
Onana will be kicked out after De Gea signed, because Manchester United can just looking for potential goalkeeper rather than train Onana.
Going back to your ex? Hell No!
There'll be nothing epic about his return. Do not forget how it was for Cristiano Ronaldo when he did return. Manchester United at the moment have a toxic fanbase who doesn't appreciate things done by legends. David should move ahead and not listen to talks of returning. Lastly, the plans of the management would be to get youngins not players with David's age. He is not returning to Manchester United.