Gold has generally had stable value throughout history, but you are right that it isn't guaranteed. Other commodities like wheat are a little harder to use to back a currency, but from year to year they are fairly stable and pretty much guaranteed to have some value whatever happens.

But US dollars aren't guaranteed to do anything for you either in such a situation. The currency could hyper-inflate tomorrow for all anyone knows, and it has no intrinsic value other than the requirement to pay taxes with it. It seems to work fine as a currency and maintain stable value, but the point is it really isn't backed by anything.

Gold has been stabilized through history because it has been used as currency by powerful governments. If the powerful governments of the world collapse, they will lose control of their large gold reserves at the same time they lose the ability to feed their citizens. What do you suppose will happen to the commodity markets then?

Nothing, neither gold nor grain, is backed by anything but power. Fiat currency is just the government accepting that responsibility de jure.