Yes, bitcoin can be regulated. Any country is free to come up with any laws they want, as long as the laws themselves are legal and supported by the lawmakers. Saying it can not be regulated is like saying the internet can not be regulated - yet it is highly regulated and there are tons of laws (like child protection) that can put people in jail.

That said, even if bitcoin itself could not be regulated, it still is part of your income and taxes are regulated already. On top of that any country can regulate what source of payment or currency of payment banks or companies can legally accept. So if they choose to ban crypto, nobody in your country can accept it.

But more importantly: Crypto is open source. Every government is able to make their own crypto coin - and you can rest assured that a majority of the people will always choose a official and government backed currency over a highly volatile internet coin of unknown origin that is hard to understand for the laymen. If a government is responsible for the well-being of over 300 million people and their jobs, they are certainly not going to base that on a currency that can loose 30% of it's value in a single day. In the case of the USA we are talking about 18.57 trillion USD, so 6.9 trillion USD gone in a single day - that would put the US back into the stoneage.

As much as i love crypto, it still has a long way to go.

ps.: if someone thinks only fiat can be taxed: wrong. If i would pay you with a car, you would have to pay taxes based on the worth of the car estimated in the fiat of your country.