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?
To me in terms of digital currency I think that bitcoin has been stradily keeping up the job. However if the definition is solely based on what you reason above then I do think that bitcoin is not a currency at par on what the world wants it to be. To me bitcoin is something that for now only the digital and the i ternet can only make sense or understand what this is all about but to the world. It is but a fugazi or it doesnt exist.