Their belief is once the number gets to what they consider sparse, their little organization will come in and either, by force of government, or by shear power of pocketbook aquire all remaining ground. By doing this they hope to be able to deny any remaining people the ability to make a living, bankrupting the few left if necessary.
Once the agricultural economy is removed, cities that rely upon farm income and property taxes will be in a drastic situation. With the proper incentives, these cities and towns can also be vacated leaving vast connected wildlife areas.