You ignore history. How many time has the world been destroied? Once, if you believe the bible, a few time if you believe in evolution. On thoes super rare points in time, gold and silver would have been fairly usless. Now how many time in history has there been a national crisis conserning the money supply. Looking at other countries, hundreds, thousands maybe? That is far more likely than a global event like you are thinking. Even WW3 would not cause PM to go to 0. Just read some history.I fail to see what the density of the metal has to do with anything, or where rareity comes into the equation.
Where I live camel **** is "rare", but that doesn't mean I want some, and exactly how many uses does a piece of precious metal have?
You can't eat it or drive it, use it to bathe in or build a new shelter.
You could use it to beat your enemy to death if it were big enough, but that would not be the prefered method, or you could use it to impress the lil woman but if she is cold and hungry and cranky as hell (which she prolly will be as we are talking about a WTSHTF scenario here) then she will most likely shut you off for a month for being a f/kin idiot.
The whole point of trade at this point will be "you have something I want/need, and vice versa".....so yeah, I got plenty of ammo and you don't, but you just ripped off the local breadmaker and I'm starvin- so i'll give you 20 rnds of .223 for a loaf of bread.
The "exchange medium" as you put it will come into play when the dust settles so to speak and sunlight once again filters through, as the thermonuclear winter begins to ebb, likely many decades after the catalyst that caused the situation has been forgotten.
By then the herd will have been thinned, life giving neccesseties wil have become a little more available and commerce may ensue as the lucky capitalist desires none of your other "possessions"
Until that point, you can put your shiny piece of metal where the sun don't shine, and don't dare come asking for my twinkies or beer.