GS, I expect that as long as the grid stays up, and electronic $$ transfers remain operable (ATM banking still working), most people will stay with the present system, even under negative interest rates, and accelerated decline in value. Any other way of conducting everyday business would be too cumbersome. To the extent that they can, they’ll try to compensate for NIRP, and devaluation, but they’ll put up with it, because they’ll have no other feasible way to go. Sudden abandonment of the $US as reserve currency would cause devaluation panic, as the Venezuelan currency collapse did there. But those who still have Bolivares, still spend them where they can, albeit at an extreme disadvantage. So too, the FRN, electronic or paper.
Grid down, or even just electronic banking down, for more than a few days, would be a very different order of business. After the riots are suppressed, or burn themselves out, the survivors would need to trade with each other for continued living, and they would eventually gravitate to some recognizable luxury good as the best intermediary trading stuff. Goods that are perishable, or consumable, or too bulky or low in unit value may be useful in themselves, but difficult to deal with as a trade medium. For trade other than direct barter, people need something that doesn’t take up too much space, or require a lot of handling, or is hard to conceal or guard. That’ll be recognizable gold or silver coinage, probably initially trading at the last recognized $$ equivalent, and rising, or falling over time, as best suits the local need.
I’d expect that under relatively crowded city conditions, a rare coin dealer could do business simply verifying coins presented for evaluation, charging a small percentage of the valuation to be verified. Merchants in the outlying areas would need to develop such capacity in-house. It would require a balance and weights, a micrometer, a magnifier, a book of coinage specifications, a secure building, and (most likely) an armed guard.