Ultimately, the inevitable clash is between Christianity and Islam. Perhaps the long-term play of open borders is a combined US/Mexico with a population over 400m to stand as a bulwark against failing Europe and provide the last defense of Western Civ.
It wouldn't have anything to do with the money associated with illegal labor, the government’s eternal desire for a new compliant underclass, unions that see the dollars of more membership, money associated with the open border trafficking in drugs, illegals, fill in the blank, Mexico's corrupt elite relieving themselves of their pesky demanding peasants, businesses profiting by having more warm bodies in the country, real estate firms that cash in on the population explosion and urban flight, banks that umm just cash in.
Nah. Too Obvious.
You give our government and the powers behind it WAY too much credit in being able to take their eyes off the dollar signs long enough to be concerned about world changing developments. Much less cobble together some twisted master plan for the future clash of civilizations. They can hardly keep up with the money flowing into their coffers to care much about anything other than money and power in the here and now.