The others can't win a vote of no confidence all the time Corbyn is there. Too many moderates will not side with him.
The correct answer is for Johnson to either refuse to take the bill to HM or to recommend no assent. The parliamentary remedy for which is a vote of no confidence which will lead to an election.
Maybe he'll get a 3 month extension, lose the vote of no confidence and set the election for 1 February. He gets to set the date doesn't he or is that only if the FTPA is used?