We can't get the toothpaste back in the tube and grow crops on paved over and built up land if by chance we suddenly need it.
It is something to think about anyways.
We are losing far more farmland to simple retirements. What we lose when that happens is the knowledge of the farmer of that piece of land as well as the infrastructure that served his business (equipment mechanics, local suppliers, and experienced field workers). What also usually happens is rapid weed infestation. Webb Ranch under the Pacific Coast Land Trust (North of Santa Cruz) was a classic, you should have seen all the hemlock and thistles. Now they plow it every year and don't grow anything. It's sickening.
As far as I am concerned, civil defense services is a perfectly rational business for a farmer adjoining an urban area. All it would take is for government to respect those contracts and not commandeer the property. See? Now you know why the service doesn't exist: government has a monopoly on CIVIL defense and the right to commandeer the farm (as if bureaucrats could take care of a mass of desperate people on a farm). Think about how stupid that is.