> My point was that if the central bank behind a currency goes away so does all of its utility, even if the currency is still physically or digitally there.
That's not true. Unless the bank is critical for conducting transactions, their presence is not required for a currency they produced/backed to have value.
Value in currencies is whatever people assign to it. Utility is whether people will accept it as payment. If the US government goes tango uniform next week, and I and those around me have enough cash available (let's also assume the banks and CC companies we rely on for card/digital transactions also disappear) we can use the remaining physical currency as our method of denominating our local economy, and likely would.
