First of all, there’s no reason to think that pressure for illegal immigration from Mexico won’t increase again; in fact, the total illegal population has already stopped declining and may have begun to grow again. The U.S. economy is bound to pick up at some point and the newfound middle-class status of so many Mexicans, often cited as a reason for the drop in departures northward, is tenuous. As the Washington Post put it, the country’s new middle class is “fearful that recent gains could be lost in a financial crisis or social upheaval.”
What’s more, the decline in Mexico’s birth rate, which is also cited as a mark of the permanence of the drop-off in emigration, isn’t necessarily connected. The change is real enough; as recently as 1970, the average Mexican woman had nearly seven children, whereas today the number is barely over two. But emigration isn’t just a matter of excess people overflowing into another country. South Korea, for instance, has one of the lowest fertility rates in the world, not to mention a First World level of development, and yet emigration to the U.S. (mostly legal) continues apace. Immigration from Japan and China seems to have actually increased as fertility declined. And immigration from Russia isn’t stopping even though Russia’s total population is actually declining. Sure, if fertility is low enough for long enough, eventually you’ll just run out of people. But in the meantime, immigration is driven by networks of friends and relatives and employers rather than by population math.
And finally, some part of the decline in new illegal immigrants, and the departure of those already here, was caused by the very enforcement measures the anti-borders crowd wants to dismantle in light of the new numbers.