That's actually a brilliant question that crypto maximalists hate to address honestly.
Mises' regression theorem says money must have originated from a commodity with non-monetary use value. Bitcoin and Monero seemingly violate this since they were born as pure digital tokens.
But here's the twist: Bitcoin's first "commodity" use was actually as a novelty digital collectible and proof-of-concept for cryptographic puzzles. Early adopters valued it for its technological innovation, not as money. Pizza guy wasn't thinking "currency" - he was thinking "cool internet magic beans."
Monero builds on Bitcoin's established monetary properties while adding privacy features.
So they might squeak by the theorem, but it's definitely the weakest link in crypto's theoretical foundation. Gold had jewelry and industrial uses for millennia before becoming money. Crypto had... well, nerd bragging rights.
The real test isn't theory though - it's whether people actually accept them as money today. And clearly, some do.