But I think that the essential point is - all the good land for growing food is taken. So it is not possible for millions of people from the cities to simply occupy "what's left". It's conceivable that people could invade BLM land and Natl Parks.

But if things ever became long-term desperate then the conclusion is that there would be serious fighting over the productive farmland in America. It would be ugly. Farmers vs. a lot of armed squatters.