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?
Among all these criteria, I always believe that Bitcoin fulfills everything well, with the exception of a stable store of value. But, is the value of fiat currency always stable, I don't think so. Therefore, it is very unfair to say that Bitcoin does not meet the currency criteria because of its value not stable.