Teams: Feyenoord, AC Milan
I don't know. I wouldn't attribute Wijnaldum's trajectory just to PSV. He's kind of a late bloomer, the level he's reached now. It's not easy to attribute someone's level to specific points or periods of time in their careers. Not that many real talents in the first place. Castaignos certainly was a disappointment, and Vilhena is ok enough but still looked like he had potential for more. De Vrij's done well, and Karsdorp I think was unlucky with his injury. I still think he's very good. The rest, not sure if they ever were such big talents (though Clasie looked good in his early twenties, but that was probably at least partly because of El Ahmadi).
But I think there's two things going on probably. Ajax has had a really good generation of youngsters just now, or recently. Feyenoord hasn't really even produced that many talents in the first place, and many potential ones have been poached by large European clubs at a very young age.
About Kökcu, time will tell. Looks like he has the footballing intelligence and technique to become quite good. I think he's too slow still, or can't compensate his slowness enough with other qualities, and is inconsistent. He's very young and has a lot of time to develop.