Crypto-only economy is nothing more than the libertarian day dream. When some countries ban cryptocurrencies right now, it isn't possible.
I will say that some do not prohibit. Most countries have declared cryptocurrency illegal. Could it be because it’s quite difficult to tax transactions? But the most interesting thing is that the officials who banned it themselves own bitcoins. At least in my country.
Even "most countries" had, banning things when you can't enforce the ban is a fool's errand. You may even argue it would help adoption.