Successful currencies meet three criteria:
(1) they are in widespread use as medium of exchange
(2) they serve as unit of account
(3) they are a stable store of value.
Most sovereign currencies fully meet all three criteria.
What about Bitcoin?
IMO, bitcoin has been used widely even for now its capitalist value is the largest in the world. Bitcoin already has an account unit which we usually call a wallet (ATM online), if the government wants to legalize it maybe bitcoin's physical ATM has spread everywhere. If, bitcoin's compared to fiat currency of course the fiat currency's more stable. But actually, the value of fiat currency's unstable because its value still depends on gold supply and demand.