I hope he can have a proper go joining the first team, but I do think there's something to be said about letting him develop in a place that he's comfortable and happy to be. There's no way of knowing if he went to a lower level PL side if he'd bed in and get the minutes he needs.
He clearly has a good relationship with the fans and manager at Sunderland, so I don't think it'd be a waste for him to have another season there.
The only times I've seen him play were the Dutch games a couple of weeks ago and he was... underwhelming to say the least. Made no saves against Italy (3 shots on target, 3 goals, goal 2 and 3 I reckon he should have done better) and 1 save against Croatia.
Not sure if that's just because Netherlands were crap, but he did come of his line rashly, flapped at crosses and I reckon he would have saved a couple if his positioning was better.
His stats at Feyenoord are a lot better, but the quality is clearly a lot lower than the 2 finals. His stats in European competition weren't as good either.