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?
Remember coins like Bitcoin is not truly anonymous. Everything can be tracked in blockchain and I'm sure government will do everything on their part to run after people who doesn't pay crypto tax. There will be strict penalties for sure.
Also, if something like ETH becomes big, how is the government gonna stop people from making pyramids, ponzis, and scams in general?
Regulations. But then again it can only cover a certain portion and not everyone is free from ponzis or scam. But they will kept reminding us, so they have done their share. Its really up to us to whether we can be a victim of this criminals or not, used our common sense and due diligence or better yet not to invest on them so that we will not be another statistics.