> It's more like a speculative investment that just happens to be easy to exchange

That's what money is. Being easy to exchange is the main ingredient.

It's slightly more convenient if the value doesn't have large swings over short periods of time, but there are government currencies that aren't immune to that, and that only really matters if you're holding large amounts over significant periods of time. Which is to say, it only really matters to speculators.

Especially if the thing actually is easy to exchange, because then it's only speculators who hold it for non-trivial periods of time at all. Suppose you could exchange stocks for cryptocurrency and then cryptocurrency for goods and services over the internet, both in less than a second. If you're inclined to invest in stocks rather than cryptocurrency then do you care about day to day volatility of cryptocurrency? No, because you'd never be holding it for more than a second even if you use it to pay for things.