The coach is really... well, bad. He's been assistent of Advocaat in Russia at Zenit and that's how he became a rich man. But he hasn't any record of training clubs himself

So he took the decision to play with eleven other players. And still, if Holland would've played with the best 11, I think Spain still would have won. Midfield normally consists of Strootman, Maher and Van Ginkel. Van Ginkel is a kind of box-to-box player, but he's the one who plays as defensive midfielder, he doesn't know how to cope with that.
About all those talents; yeah it's frustrating. But actually all those clubs do this. Feyenoord also pick talents from other clubs in Holland. Ajax actually is doing this the most. Let's look at their 'self-educated' players and at what age they came to Ajax.
Eriksen (16), Alderweireld (15), Vertonghen (16), Vermaelen (17), Fischer (17, 1.3 million), Andersen (17), Meleg (19), Siem de Jong (16).
You can't stop youth players leaving your club. It's up to the club to give those players a reason to stay. Bruma for instance would have played for two season at Feyenoord already, where is he now? On the other hand Rekik has developed well at Manchester City, Ake gets his chances at Chelsea. It's not necessarily bad for their career.
Feyenoord had a great youth squad including Vilhena, Achahbar, Kongolo, Boëtius (still at Feyenoord), Rekik (City) and Kyle Ebicilio (Arsenal). Those guys played in one team and two decided to leave. If there are enough players who decide to stay, it still benefits a lot to invest in the youth. Vilhena for example has had offers from big clubs in England and also Juventus, but he decided to stay. Same for Achahbar.