If she gets rid of the backstop completely, MPs think enough will vote for it to pass. If she doesn't it won't. But May regardless is on borrowed time as she is being ridiculed & called incompetent by everybody including her MPs and Tory party members & many commentators are also saying she is probably the worst PM ever. She has used up beyond credibility with all her threats to MPs, her postcard to the public which Tory party members have refused to deliver and her waste-of-time cabinet roadshow, saying all along that she won't cancel the vote only to do so at the very last minute.
MPs will then let her own it until post 29th March 2019 and then get rid of her. She has done far to much damage to the Conservative party and where the cabinet & MPs think they have then got off scott free once they get a new leader, they haven't as the Tories have totally lost their tag of competent government & many will not vote for them again even if it means a Corbyn government.