For that to happen, living in a dense urban area needs to be much less expensive than living out in the middle of nowhere. The average income in Mississippi is around $25k / year. I'm not sure what the distribution curve looks like, but I'd guess 2/3 of the population make at least $20k / year. Cities should be designed so that a person can live a good life on $20k / year in a city.
I'm trying to think of places where city living is significantly less expensive than living in more rural areas and about the only thing I can come up with are places where land is scarce and therefore expensive. For example, Hawaii.
And how do you propose to get produce if all the farmers move away? Saying "people should move to more dense areas" creates a whole other swath of problems to fix.
