Food has pretty much the most inelastic demand curve imaginable. There's no way that agriculture will disappear without subsidies. Prices might increase, sure. But anyone who has any money will pay whatever prices are necessary when the alternative is literally starving to death.

If you're referring to agriculture disappearing from the US, that definitely wouldn't happen either. Some of it might, but not the majority. Most countries don't heavily subsidize farming to the extent that we do, and transportation costs are non-trivial, especially for fresh produce or anything that needs freezing.