Mexico is playing this all wrong.

When you only get dealt half a hand you don't play for a win. They might also actually entertain the current system -as beneficial as it was - was never going to last no matter how much they might like to think otherwise.

Better approach would be to pay for the wall - offer to fund it with a small tariff on exports as a negotiation concession on retaining NAFTA status. Build it to moderate standards with Mexican companies and labor.

If you are going to lose - lose smart.