De Gea leaving for Madrid means feck all. It certainly doesn't make us Madrid's feeder club, that's a ludicrous thing to say. For all we know the reason could be his girlfriend wanting to sunbathe her ass on the Madrid streets (and it's probably true).
Ronaldo and possibly De Gea leaving for Madrid are the exceptions, not the rule. It's just unfortunate that it's happening for the second time in 6 years.
We're up there with Bayern, Barcelona and Real Madrid in terms of stature. People should accept the fact that sometimes key players will leave us for Madrid (Ronaldo, De Gea) and sometimes key players will leave Madrid for us (Di Maria and possibly Bale).