Craig and Bruno are "fixtures," they'll be there as long as they want to be.
The BBC likes some continuity. As far as they are concerned, a case of, "if it ain't broke don't fix it."
Darcy, could be replaced, but only by another female. She presently fills the "we must have a woman on the judging panel" requirement. To me, she seems no more important to the show than was Alesha.
So it is possible that there will be two new faces next year, if the BBC want to "freshen up" the panel.