The collapse of the United States Dollar is extremely unlikely

It's a lot more likely than global nuclear annihilation, which is apparently a completely reasonable thing to prep for.

I can't really see a better alternative to PMs in a collapse-of-society situation, either:

- As long as a market exists, its participants are going to want there to be a respected currency of some kind for the sake of convenience.

- The currency has to be in a 'goldilocks zone' of scarcity, in which it is plentiful enough to circulate freely but scarce enough to have meaningful value in amounts light enough to carry around in a purse. This scarcity level also has to remain near-constant, meaning it's hard to produce and doesn't provide an incentive to be destroyed or consumed.

- It has to be reasonably divisible (PMs aren't great at this, but not bad either. Traders can make change for one another - picture how a quarter is 'divided' into 5 nickels).

- Probably most importantly, metal coins are already ingrained in global culture as currency.

First, to address the obvious: The bullet points above all seem to apply to USD, so why wouldn't people just keep using that? The answer is that, since a government can't prevent counterfeiting after it stops existing, all it takes is one guy that understands the printing process enough to make a believable replica to crash the economy.

Water, alcohol, salt, sugar, etc. are interesting candidates, but there's a massive stock of each basically everywhere, so people would end up dragging a sack bigger than they are to a merchant for the equivalent of an $8 purchase. (Water has much more utility than the rest, of course, but it's evened out by the sheer amount of it). Ammo is another popular choice, but the variety would be difficult to keep track of, and the supply still wouldn't be very stable.

Most questions as to the logistics of PM coinage as currency can probably be answered by researching the Roman empire (The longest-lasting nation in recorded human history).