He plays for the national team, and that's all that matters. I don't want a team that's coming close to half of it from one foreign nation, its bad enough that in England we hardly have teams that have a majority of English/British players, in my mind it don't fit 100% right. Plus if Slot fucks it up next season and gets the sack then what about the next manager ?
There is also something else - I remember reading the late Bobby Robson's book, where he described Dutch players as having a bit of an attitude if they don't get their own way. The actual Dutch national team over the decades has been known to implode with internal issues amongst players/coaches - trust me - its not a good idea.