They cannot swap him out because he will not cooperate. Attempting to swap him out would do nothing but split the vote for them.
Repubs have spent decades propagandizing their fear-addicted voters with racist delusions. Donald has taken over that mechanism.
The sane but sociopathic Repub leadership is experiencing the classic trope of a monster they thought was tame (the racist voters they have been agitating) turning on them (a dementia patient they have no control over blathering about eating cats during the debate).