Governments make money on the movement of money. For this reason they encourage all types of money movement from spending to investment, much of the latter is encouraged by adjusting interest rates so that wealth needs to invest to gain or lose out to inflation. It doesn't matter if it is a state run fiat currency like the USD or a cryptocurrency or a foreign currency or any type of gain that isn't a loan, gains and money movement are taxed and will always be.

Taxes are collected on spending (sales tax), on earning (income tax), on gains from investment (dividend/capital gains) on gifting and more. Governments are there to collect on those areas no matter the currency.

If anything cryptocurrencies are a hedge on state issued fiat currencies in case they crash or ones that stop transactions, that doesn't mean money isn't moving and that doesn't mean governments aren't there to collect. The IRS sees no difference in USD or BTC gains or any other currency.

Money laundering is also part of even very well controlled fiat currencies, sure they might get the small fish but large fish are money laundering in real estate, art, political campaigns (with Citizens United), offshoring hoarding, offshoring investment, banking fraud and all sorts of fronts even in fiat. Cryptocurrencies also have that aspect but every currency known to humans will have some amount of money laundering. You might argue that many benefit from this type of behavior as well including banks, real estate and more regardless of currency. If anything money laundering increases market values, up to 30% of high end real-estate even with fiat currencies or the USD is suspect to money laundering. [1] There are also major holes in hoarding with our current fiat currencies in findings such as the Panama Papers. [2] In a way, money laundering is bad but in another way money laundering legitimizes money and that money goes on to be spent, thus some collection of taxes eventually happens down the line, hoarding or discouraging investment is the real problem.

The only real issue of a cryptocurrency would be hoarding and a non fiat currency that is constantly increasing in value that would discourage spending and investment. Right now fiats have interest rates controlled so that they lose value to encourage wealth to invest to retain and advance their wealth, money sitting in fiat currencies is slowly losing value due to inflation. Investment must take place to gain through creating revenue streams and collecting interest/gains on investments. Poor people pay interest and rich people collect it, that is the compounding difference in wealth almost solely regardless of starting wealth.

Fiat currencies and cryptocurrencies all are gains and money movement that governments will collect. In a least case scenario, taxes may have to shift to more sales tax and VAT tax type of systems but governments will get their cut of the money movement no matter the currency, nothing really to worry about. The argument of too much money laundering is also moot when things like the Panama Papers and offshoring exist and are allowed, or the bulk of money laundering in real estate, art, political campaigns and more via shell companies and foreign infusions whether incoming or outgoing.

Hoarding and inequality is the major problem as it leads to stagnation and lack of investment, money laundered eventually becomes legitimate and is the money re-entering the system which isn't always bad. Right now we have a hoarding problem or in inequality problem that is worse than money laundering, the latter which legitimizes money again to be taxed somewhere down the line. Right now the M2V is at all time lows, money movement is stagnant and that is a major problem [3]. If governments and economies make money on the movement of money we need to do better, crypto helps that problem rather than exacerbates it.

Even if there are more 'hidden' transactions in cryptocurrency there is still movement into income for income tax, spending via sales tax and transaction fees. You'll never have to worry about the government getting their cut, they will get their cut. What you should be worried about is money movement in the lower/middle class which leads to stagnation if hoarding and inequality are prevalent, which in the end harms all including the wealthy.

[1] https://www.cnbc.com/2017/08/24/a-third-of-luxe-real-estate-...

[2] http://www.newsweek.com/panama-papers-finance-business-mossa...

[3] https://fred.stlouisfed.org/series/M2V