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?
That's the reason why the government won't allow a decentralized cryptocurrency to take over fiat because they wouldn't be able to control it. Unlike what is happening with fiat wherein they can control and trace the people behind the transactions, in cryptocurrency, only the addresses are easily traceable but not the people behind it. Government would think ways to still control it like creating their own cryptocurrency perhaps.
This is true, government needs to find ways to receive tax revenue if fiat money is to be discontinued