Whether or not crypto is determined to be money or not it is still personal property which has value and can therefore be subject to taxation, anything which produces a benefit can theoretically be taxed. The important distinction after a government does deem it taxable is if crypto is a currency or a commodity as there are different rules, store of value, fungibility, unit of account and so on
But how will they levy tax on crypto currencies if they doesn't know who to tax? They can't actually tell whose using Bitcoin and whose holding them. Even if they write a law making these holders pay taxes for their cryptos, they can evade that tax whenever they wanted since they don't have a proof that they really have bitcoin.
I know not all of us would be doing that but I know that will happen if that is the case. There are a lot of taxpayers that evade taxes, how much more when if its bitcoin.