One can very plausibly argue that Trump just wants an end to NAFTA so that he can tell his base promise made promise kept.  Some on his demands are clearly non starters for Canada and Mexico.  A sunset clause, for example, would mean unending negotiations and demands as well as unacceptable uncertainty for businesses in all three countries.  And chapter 19 is necessary to avoid ongoing America first rulings from their courts.
All negotiators ask for more than they expect to get but these can of demands are ludicrous and just waste everybody's time.  If he wants to cancel, time to do so or get off the pot.