Bitcoin does not need to be a currency for them to be able to tax it. If you sell a house and you make a profit of say $100 000, the government
would take their share of the gains you made in the form of "Capital Gains" taxes. (Each country has it's own guideline and tax exemptions) So what
most governments are doing is to class it as a commodity and thus having you paying "Capital Gains" on the profit when you sell it. The main reason
why they are not defining it as a currency is because they do not want it to compete with their local reserve currency.

Some countries tax
Bitcoin as a currency (VAT) and a commodity (Capital Gains)

Heres where the problem lies for me, how do you pay taxes in fiat if you don't have any fiat? If I trade my Ethereum for Bitcoin, and am now holding Bitcoin, how am I supposed to pay taxes on that if I don't hold any fiat? Can I pay with Bitcoin? In most places NO.
Also when it comes to swapping, lets so my I have a beer in my hand, and my friend has another beer, but I want his beer so we swap beers..............do I have to pay taxes because his beer is cheaper than mine? Swapping Crypto is no different.