Nope. It isn't unusual for players to play for both Madrid teams. I would be surprised if Madrid doesn't come for him in a couple of years. And I would be surprised too if De Gea wouldn't be interested a bit (and much more if we are still struggling).
Ok I get that. Even Real's greatest modern icon Raul signed from Atletico after their youth teams shut down. But I'll again say never in a million years will De Gea move to Real. Those players weren't lifelong fans like him, they most probably didn't come through Atletico's academy as kids and they certainly don't feel so deeply for Atletico as De Gea did when leaving the club. These were his parting lines :
"It’s been just over a week since I signed the contract with my new team. It seems like yesterday when I was four and joined for the first time Atleti in Casarrubuelos. From the plane I travel in to join Manchester United I remember with nostalgia, that this nostalgic feeling will be my traveling companion for a long time.
Like any child, every evening was accompanied by excitement and my parents (without their support none of this would have been possible) with the sole aim of defending the goal of Calderón someday.
A little over a year and a half I completed my dream and sealed it with two titles, I will never forget that feeling, I will never forget the people’s faces painted red and white. I can not forget neither the tears of Barcelona, behind the chants of support for us.
Today, 16 years later, I undertake a new challenge with the same enthusiasm of the boy and the eternal gratitude of a feeling, that only we understand, that I will carry forever in my heart regardless of the badge that adorns my chest.
I see from the airplane window how I leave behind my house, thanks to my new club for the opportunity it provides me with the security there to finish my education from a distance, that they only put the miles, you are proud of that boy who dreamed that the Calderon would sing his name.
A thousand feet above the ground I remember, now more than ever, all the teammates, coaches and club workers who made it possible that the child is now a man.
Winning a title there is a mix of smiles and tears. This is what happens to me now, tears because I am leaving, smiles for the illusion of something new, but also the assurance that someday I will return home. Thanks for everything."
He has deep roots with Atletico. The fans still remember his services to the team. They even have his supporter club banner in the Calderon :
Do you think he'll spit on all of that and sign for their bitterest rivals ? Not a chance. It'd be madness along the proportions of Gary Neville signing for City.