Volatility is an an inherent property of an asset/currency/whatever(stddev of (log)returns), as long as you have a fixed and distinct numeraire.
Sure, you can argue that bitcoin may _be_ the numeraire one day, but what's the path towards that when it's so useless as a currency presently?
Either it's good as a currency, or it's good as a savings vehicle.
You seem to be confused about which line you're pushing
Of course there is, for example it would be necessary for it to be useful as a medium of exchange. Which, bitcoin certainly isn't.
I can't purchase, for example, >50% of the basket of goods that comprise the CPI with bitcoin.
Sure, you can argue that anything which can facilitate more than 0 exchanges with more than 0 counterparties is, at least ephemerally, a currency. But if you're arguing that, you've already lost.