Just because the blue cities break away doesn’t mean they will give up eating. What happens is that the Blue cities will have to compete with the rest of the world for that food. As already pointed out the cities are the devil to the social corruption we are facing. Expell them from the union and treat them at arm’s length.
There has been no civilization without cities- that is where the word comes from. Any civilization that loses its cities go into a Dark Age where international trade and literacy collapse.
Agriculture needs the cities as much as the other way around.
A farmer’s family can on eat so much corn and if he can’t sell the rest he goes bankrupt.
Without the cities the economy would collapse, they are the economic generators of society as well as the creators of new ideas.