This opens up the prospect of a regency, (possibly an unpopular one, depending on who Billy marries) which can't be too bad for Jacobitism.
It's just as well that William's maternal grandfather was Charles I, or his potential dynasty in this situation would have a really shit blood claim to the throne. (I.E, none) The whole reason he was King, after all, was purely that he was Mary II's hubby, let's not forget. (Parliament didn't even want to make him King regnant at first.)