Dave is not leaving. We'll get the contract ironed out and he'll stay here for the long term. How many world-class keepers at top clubs are transferred anyway ? Closest I can think of is Buffon from the then lower elite Parma and United are way bigger than that.
The likes of Madrid are impersonal commercial operations (like even Ruud alluded to), and while we're headed towards that kind of a structure going by recent developments, United is still very much of a family club. And David genuinely seems to love the club and the supporters. Lots of players are disingenuous about it but one gets the sense that he truly means it. We bought him at a really young age, the club supported him through the rough times and he genuinely seems to appreciate that.
The only place I'd imagine him leaving for is Atletico in the latter stages of his career, he even said in his farewell note that he'll always have the club in his heart and will return one day. People don't seem to realize how much Atletico means to hardcore Colchoneros like De Gea whose granddad and father and family are Atletico ticket holders. The guy was there since he was a kid and having a little brother syndrome, Atletico supporters made a conscious choice to root for the underdogs, they detest Real, its capitalistic values and everything the club stands for. There are some massive historical divides between the two clubs, leaving one for the other is considered treason.
He is a relative legend at Atleitco, why would he throw it down the drain and risk the ire of his fellow Atletico fans just to force a move to Real ? Makes no sense, we could probably make him one of the Top 3 highest paid keepers in the world. Reckon Madrid will take Cech off Chelsea's hands and that'll be that. Barcelona already has Ter Stegen so it wouldn't make any sense to target De Gea post the transfer embargo either.
Unlike raul who went to real and became a legend
Raul left primarily because Atletico's academy was shut down, not because he wanted to be a Real Madrid legend or personally sought a transfer. Also he wasn't an Atletico product, was there for just 2 seasons after transferring from San Critobal. David in comparison was there for a decade, starting at age 10, is a hardcore Atletico fan and went to see their matches at age 6 with his grandpa. Totally different cases.