There is no true "momentum" in the U.S. to end NAFTA right now. Many momentums  at this moment - including the large bulk of its business world realizing that the trade deal is a key to their prosperity. Multiple northern states rely on Canadian trade for their standard of living. Given the grand political troubles Trump has already met during his fights for health care and tax reform, I don't see the formal "ending" of NAFTA actually happening with out a much louder and divisive debate in Washington D.C.. More politicians yelling and screaming - including Republicans yelling at each other loudly. NAFTA is a key to American prosperity too. As you said, Canada and USA could fall back on their previous agreement. Let's hope we do if these negotiations go nowhere.