Well, a lot of countries are taking a hardline against crypto since it's threatening their tax revenue like you say. Furthermore, investing in cryptos means a reduction in demand for the various sovereign currencies (Euro, dollars, etc) while simultaneously increasing supply. A mass adoption will also make it hard to implement fiscal and economic policies, meaning the government will essentially lose control of the economy.

A hardline against cryptocurrencies will only increase its decentralized usage since the only way to get some of the currencies is through p2p exchange rather than through the various exchange like Coinbase, etc.

Banning cryptos is just a small dent in the overall long term development of money.