You described effect observed in population that grew up in, and descended exclusively from generations living in singular over-crowded space with full living n

You described effect observed in population that grew up in, and descended exclusively from generations living in singular over-crowded space with full living necessities supplies. Humans out in the real world aren't like that. Some live in the crowded city, some live in the open regions. Some hunt for best deals in the mall, others hunt for the game for lunch. There's a constant trickle of cultural & genetic mixing, and there's a lot of small towns etc. that are somewhere in-between on the scale of self-reliance. If the city populations were to crash, we the humans wouldn't really suffer all that much. Perhaps less new entertainment content would be produced, but the necessities would be taken care of as per the usual, knowledge would be preserved, and communication & trade would go on. In due time, cities would be re-populated.