BTC monetizes the infrastructure of anonymity, but does not enforce transactions. It's short of key differences between a token and a currency. Fiat is the ability to create currency, but fiat does not come by fiat. A real currency is one that can command an economy capable of enforcing its will. This is why the money of many countries is called "junk" currency - the country can't actually enforce its writ.
Computer programs expand so as to fill the core available.