The degree of usefulness of a medium of exchange, is measured by its recognition as such, nothing more You can try to offer gold in exchange of something to hypothetical incas, if for them gold is worthless because they shit tons of it every morning or whatever reason, then that's what your gold is in that environment, worthless And I don't think the safest areas will accept seashells as a medium of exchange/currency Because yes, there will be safer areas, the world isn't going to end overnight and go full madmax to infinity over a mere financial crisis of biblical proportions You would need a nuclear mexican standoff between NATO - Russia - China for that to actually happen