In theory you are right, governments can really stop cryptocurrencies, in the same way they can't stop crimes or illegal trades.
But if the use of cryptocurrencies become too dangerous, its use will be very limited just to illegal trades, and it will lose most of its value.
There is no way for a government to freeze a crypto account, but - or course, - it can make illegal to exchange it in fiat money.
But even in this case you can make a lot of things with your cryptocurrency (you can go in another country, for example, or to exchange them personally)
We'll see.