One problem with this idea is that the land usable for agriculture also has to compete with other uses: residential housing, etc. You can't build housing in pl

One problem with this idea is that the land usable for agriculture also has to compete with other uses: residential housing, etc. You can't build housing in places where you can't grow food, because people don't really want to live in those places. So any place that's unusable for agriculture is also generally useless for most other human activities (it's either too hot or too cold, or is a wetland). We already have enough environmental problems because humans have taken over too many wild spaces. There is not a large supply of usable land left; we've already used it. Point to a place that's currently unused, and I'll show you why no one wants to live there.