There's also a paradox involved. So long as Mexico is impoverished and corrupt, many Mexicans this side of the border will be glad they got away and not want to bring their corrupt elites with them up here. But so long as Mexico is that way, the immigrants will keep coming.
There's something of a scam at work as well. Aztlanists play the racial angle and argue for a "brown" Mexamerica, but among the Mexico City elites whiter is better. That sort of thing is pretty typical of ethnic politics, though.
The bottom line is that "facts on the ground" -- who lives where, and how many people there are in each group -- matter more than legalistic arguments. That's as true now as it was in 1836 or 1850.
But that is in the cities. Most of the counties of the Southwest outside of the big cities have a very deep "cowboy culture." These Americans are not sissies, and will not go quietly.
We will be in for a Bosnia type civil war, with the invaders controlling the cities, and the Americans controlling the land in between. It might become problematic in the cities, if, for example, their pipelines and powerlines are continually cut off.