I don't think the 'when was the last time we sold a player to Real' argument is as valid as people make out. As I said earlier, in my opinion, it is almost seen as a thing of pride to have a player that they would a actually want. The fact is, even most of our beloved heroes we have had in recent years may not have gotten into their team, or they would not be interested for other reasons. When we had the De facto best in the world, there was an inevitability he would go to where the best players (officially ) in the world tend to play - Real Madrid (or Barca). Fans seem to be approaching the De Gea situation with the same inevitability, despite minimal noise from Madrid themselves. It is almost a case of 'he's so good, he will obviously end up there'.
I remarked around the time of the Januzaj hype last season when there were question marks about his contract at how many posters were saying he should sign for us now, and then he can go to Madrid when he's 25 if he wants. There had been nothing from either the player or Madrid to even hint of an interest, and we could not even play the 'all Latin players dream of bla bla ' card with a Belgian. It was simply inferiority complex speaking I think, and the expression of many of our fans seeing us as a step down to Madrid, and a place that Januzaj will of course end up if he becomes so good that he may dare to catch their interest. At least we an get some good money and grab some bargains from their boot sale as part of the deal hey!
Likewise , I have seen justification on spending X amount on Pogba as he is still young and can be sold to Madrid for profit once he becomes the best in the world.