Fiat currencies are like Unicorns and Santa Claus, they stop existing when people grow up and stop believing in them. And there is no need for a "replacement" (i.e. the nothing is better argument) or a need for "enforcement" (i.e. the military argument) or a need for a "system" (i.e. the rule-based-order & courts argument). Fiat currencies simply die off due to lack of trust and belief, and it often happens by way of Gresham's Law.