Money is an agreed upon tool for exchanging items/services of value - but it also a store of value, because it retains at least a semblance of stability. Cryptocurrencies are... well, not currencies, for starters, but they're also simply a proxy for "real" money (ie: fiat).
Where cryptocurrencies entirely fall apart is when the realization hits that they all fall into the greater fool category - once you run out of new marks, the scheme collapses, and all the money - note actual currency - ends up in the hands of a few early adopters and a couple extra lucky folks who timed it right. Negative sum games are not a good thing to prop up.