Farming productivity levels would go back to around 1800 or so, and we could barely feed a billion people at that time.Agreed. That's why each community would have to do homesteading and regenerative farming on much smaller and more distributed scales like we used to. All centralized mono-crap farming would be gone and cities would have to break up and move into rural communities. People would need to have more kids to do work around the homestead. People here would know this concept as "horizontal scaling".