How do they know? In 2050 the USA might not exist—it could be a part of East Asian Co-Properity Sphere or something.
I'm acquainted with a few who believe it no longer exists now, we're just being strung along, placated and slowly acclimated. To them, that's why the "border" is not being enforced, it's no longer there except as a fiction to prevent native unrest. It's also why so many Presidential contenders of late have been dicey as far as eligibility under the natural born citizen requirement. That would have to go in a post-national world.