If they do, the EU project is over. Ever other country is the EU will know that they just have to threaten to leave and they'll get their way.
Remember as well that the EU people don't like BJ. Most people don't. Politics is about people as much as policies.
One theory is that BJ is threatening 'no-deal' as a gambit. Ultimately, he'll try to settle on a 5yr exit with no NI backstop. The EU will huff and puff but accept it (& fuk Ireland in the meantime). He'll tell the British people that he tried his best and they only have to put up with those EU feckers for another 5 yrs and then they're out. It's a halfway house between out out, staying in, and the legal 31 Oct deadline of automatically leaving.
N.B. He's not officially meeting the EU people until 2 weeks before the legal deadline of 31 Oct. Whatever he agrees, he won't get it through Parliament in 2 weeks?