If crypto does gain massive adoption and you are able to pay for things in crypto, there would be less of a reason for people to cash out into fiat. How would government collect taxes from crypto gains then? Also, if something like ETH becomes big, how is the government gonna stop people from making pyramids, ponzis, and scams in general?
They will seek to develop software to manage the crypto market. As you see, the crypto market contains a lot of people from different countries. A government will not be able to manage it all. They need to re-union to have the best solution.