There is a difference between "Survive" and "Thrive". Of all the civilizations that got sack-tapped by the late bronze age collapse, Egypt weathered it best because while bronze made life better, you can't eat it. As they were the top grain producer, everyone could still eat, but their economy crashed, hard.
The US is IIRC one of the top food exporters. The US will survive, but it'll be a rough sort of survival, and sometimes collapse has better long term outcomes for peoples/regions; the Mycenae collapsed, but from that collapse the greek city-states arose who prospered while Egypt survived but struggled for relevancy, with the Greeks eventually taking over for a time and putting thier own Pharoh on the throne.
Mexico would be a huge liability. You'd be better off just putting automated turrets on the border than trying to integrate them. US planners have known this since the 1840s, I don't expect a reverse course.
In the 90s, Canada being the 51st state in the event of global crisis was nearly a no-brainer. CURRENT YEAR, less so. Canada's current regieme has beholden themselves to non-native interests in ways Obama has wet dreams over, and would likely get dragged down trying to prevent the collapse, which could create an interesting situation on the northern border.