Fact is, if the negotiations fail, NAFTA does no necessarily get thrown out. Look at the healthcare blockage. The American Senate and House could move to prevent Trump from "ripping it up" in the interest of  protecting local industries reliant on trade. 

We could literally find ourselves a few years from now exactly where we were before this screw ball got voted in. We know that he knows that these negotiations aren't meant to succeed. That's why they're showing up at the table unprepared and unable to move the ball forward. They don't care. It's not worth the effort if it's a foregone conclusion. We should call the bluff and send a bunch of garden gnomes to the next round. 

All of this will have been done just for the glorification of his massive ego (sorry Seinfeld).