Those 15 additional seats would have to come from voters who did not vote for him, getting angry about him not governing. If they wanted him in the government, why didn't they vote for him?
And this is especially true in the case of NSC, who've said multiple times beforehand that they'd rather not govern with PVV. If you voted for them, you can't (and probably wouldn't) really be mad at them for doing what they said they'd do.