What is it about current agricultural practices that makes them unsustainable? I know there is a reliance on fossil fuels, but that seems like it could be overcome in the long term (1000 years) without needing to move to a decentralized system.
This happens with decentralized farms, too. The problem is not in the kind of farm, but with the techniques used. Tractor tillage increases yields, but is hell on topsoil.
[1] https://www.scientificamerican.com/article/only-60-years-of-...
If you're drawing on the aquifer faster then it replenishes, it'll eventually run out. Sure, it'll replenish itself - eventually - but the land you were cultivating will be unusable for decades. In the meantime, all of your nice topsoil will turn to dust, and blow away.
It's easier to conserve the soil that's already there than to make new soil. That said, there are techniques to increase the production rate of soil. Sheet mulching can create 4 inches of topsoil out of waste organic matter in 2-5 years (compared to thousands of years for natural processes).
Generally it's better to make soil "in place" because moving it breaks up important soil structures, leading to oxidization and nutrient loss.
https://en.wikipedia.org/wiki/Sheet_mulching
http://tobyhemenway.com/resources/how-to-the-ultimate-bomb-p...