To extract Gold or Monero, it require energy consumption, which is done in the time consumption. So at the end, money is energy stored over time, something like a battery. Fiat is just diluting the battery energy over time.
PM will never be worthless because of the production cost and it's ultimately the base of all the economy like it has even been. In case of Mad Max situation, PM will be used like it was before, and this will never change whatever the degree of the crisis because it's the most robust & efficient vehicle to store wealth in chaos.