If you listen to the technoliberterians on the respective subreddits, BTC and ETH will destroy all fiat and central banking. The realists realize that is highly unlikely.
At this point, I think we need to define a "currency". I googled it, and wikipedia (what else?) told me that:
"Currency in the most specific use of the word refers to money in any form when in actual use or circulation as a medium of exchange, especially circulating banknotes and coins."
Also in Investopedia:
"Currency is a generally accepted form of money, including coins and paper notes, which is issued by a government and circulated within an economy. Used as a medium of exchange for goods and services, currency is the basis for trade."
CCs are circulating as a medium of exchange; emissions and transactions are [dis]proven by automated government (consensus of participants, iirc). What's the difference? In that uncommon "government", consisting of real demos this time?
I have no doubt they will eventually be used as a class of currency. Thinking they will replace/destroy fiat currencies that have been around for hundreds of years is a bit of a stretch though.