An economically collapsed planet would force some hard solutions onto the problems of overpopulation. That is, after all, the root problem since it is the root decision made after the war, since it was politically impossible in all nation-states to tell the people the truth, that you've got to start controlling population growth because the planet's resources are diminishing.
I think this may be an excuse for a more malevolent purpose. There is no such thing as over-population, only over-concentration. Our numbers will not exceed over 5 billion. This is scientific fact. There is plenty of land AND resources that could be utilized, but they pretend that this is a problem that cannot be solved. Bullshit. This earth is PERFECT for sustaining life and a full population, yet we are kept from prosperity. We are kept weak and struggling so that we are easy to please, and even easier to control.