but i think it is already a problem for the government. because government have to turn bitcoin as legal currency first, and after that they can impost tax on bitcoin, because there is not tax on illegal thing. so if government want to impost tax on bitcoin they have to turn bitcoin as legal currency.
There are other ways. Germany, for example, considers bitcoin to be something akin to shares or other things that you can trade, not as a currency.
When you buy and sell bitcoin within less than a year and make a gain, this gain is taxed in Germany. If you make a loss, you also have to declare that, but you can count the loss against future gains. If, for example, you lose €100 in one year and gain €150 in the next, you only pay tax on the €50 total gain.
This is how Germany handles it. I don't know about other countries.
Of course, some people will try to keep their ownership of bitcoins secret, but the choke point is the bitcoin exchange. All exchanges I know are reached by the OECD AEoI/CRS regulation and have to report account information to the home taxman of each customer. Some exchanges may not do that yet, but then they are breaking their own country's law.
This leaves trading on LocalBitcoins.com and on the Mycelium Local Trader, but that is slow, inconvenient, and often costly, not to mention certain risks.
I think it is a good idea to fulfill your tax obligations within reason. The bitcoin hodlers are not affected in Germany, because gains from selling after more than one year are entirely tax-free.