In the United States, if the "city dwellers" all disappeared, the "rural folk" would go from barely getting by despite being heavily subsidized by everyone else to some kind of devolved state that looked like an Amish version of The Road Warrior.

If the opposite happened, urban areas would buy their food somewhere else and eventually save some money thanks to the increased competition in the market and the lack of agricultural subsidies, not to mention all the other ways we subsidize rural America's inefficiencies.