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?
Its not just about being adopted, its already adopted by a lot of people, the main problem is that the price is volatile and no one is ready to use a thing as volatile as Bitcoin. If it becomes stable, I believe that a lot of people would start to make use of cryptocurrency for making transactions. But whatever, everything will be the way they are meant to be, and thats it.