I don't see how DDG is relevant really, to be quite honest. Real Madrid can afford to wait another season, if that's what it takes. Just as we can afford DDG leaving at the end of next season, this isn't really a carrot and stick situation; if Ramos signs a new contract and we're banking on signing him to push the DDG transfer fee up, or using it as some sort of carrot to get Real Madrid to sell a player they want to keep we are kind of screwed. The idea defies logic, not to mention Woodward seems to be a shrewd business operator... these sorts of hodge podge deals people come up with are silly and unprofessional. Don't forget players are people so much as they are 'commodities' so to speak. If a player wants to leave, he will go for whatever price, which means DDG will go regardless of what happens with Ramos and vice versa (assuming that is both players' intention).
The likelyhood is that Madrid won't be able to register players next summer.
In that scenario you have to wonder if De Gea might sign a fat extension.
On top of that Perez likes to get his man. It would be very embarrassing if we told him to feck off... and took Ramos.