To become a Unit of Account, you generally need to become a Means of Payment. To become a Means of Payment, you need to be a store of value.

Why would you price a product or service in something without value? Why would you denote all prices in something that nobody is pricing in?

Your blanket argument that all successful currencies have been backed by a state is obviously wrong. Gold, Salt, Rai stones, sea shells all had value and were used as currency without governments taxing in them. Rai stones especially are a very interesting use case of money.