"Entirely" is the keyword.. there are certainly a lot of things you can buy with crypto, but at some point you need to convert it to a 'real' currency to pay for food or gas or server costs or whatever. And if you find a service that lets you buy food with crypto, eventually they will need to pay their suppliers with 'real' money. Everyone needs to keep track of the btc/USD (or whatever coin) exchange rate to ensure there are enough 'real' dollars or euros or whatever at the end of the day. It's a super hard thing to bootstrap a currency, much easier with the force of the state, I doubt there will ever be a truly competitive crypto.