If that happens, it will play out like this: Obrador, who the underclass prefers and sees as the "reform" friend of the little man, will be ousted in a revolution, much like Francisco Madero, who was viewed the same way, was ousted in favor of those who are even further to the left.
My theory is that a fear of this scenario above is precisely what has kept the U.S. government from closing off the border. They view the border as the only release valve for the pressure cooker that is Mexico.
Your valve theory makes a lot of sense.