There aren't too many good Dutch coaches under 70 around right now. Koeman, Ten Hag and Bosz have the highest profile, but Koeman just left for Barcelona, Ten Hag just renewed with Ajax and Bosz just joined Lyon. I am anyway not sure whether Ten Hag and Boss would be good national coaches with their style. There are plenty of coaches in the next tier (like Van Bronckhorst, Cocu, Van Bommel, and so on), but each one would be as much a risk as De Boer was, and that's probably not what the Dutch FA wants right now. So there.