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.

Thank you, this is exactly what my rhetorical question was aiming for. If BTC/ETH are investments, fine... that is a world away from currency.

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.

Destroying and replacing are very much orthogonal to each other.