It's not just a question of level, but also of type of football, culture, manager at new club, tactics and many more. Also, some players just decline more quickly than others, or never reach their potential. There are no gurantees in any transfer, not even in transfers amongst Premier League clubs. The Eredivisie's record for bringing along top class forwards is second to none imo, but not every transfer can work out, just like not every wonderkid can work out. Football isn't that simple.
As for De Jong, he was never potentially top class, but I did expect him to be good enough for our current level. Sadly, he was not.