I find the whole line of reasoning in the xkcd comic quite amusing when it comes from crypto 'shorters'. Admittedly, I am on the 'long' side when it comes to crypto and decentralization.
Suppose your government actually had to act this way in order to crack down on cryptocurrencies. Every prison would have its own Guantanamo bay for torturing private keys out of cryptocurrency users. And this is is portrayed as some kind of win or gotcha in the comic. The non-violent offenders glutting the prison system would sky rocket as judges have to hold people in contempt indefinitely for not giving up private keys(this has already begun happening in the US).
"Well you forgot to account for all the knees our liberal democracy would be willing to break!"
If that's what it takes then you've already lost. This is the private fantasy of a lot of individuals long on crypto and the broader decentralized trust-less ecosystem. Because the implied threat of force is often a much stronger form of social control than the actual use of force itself; which is expensive, unpopular, and broadly illegal in developed countries.
The ideology of this decentralized ecosystem mocks the myth of the benevolent nation-state prosecuting 'criminals' on behalf of its 'citizens'. A nation-state with the power and will to exist in such an ecosystem would be the most cruel and invasive authoritarian regime in human history. Any reasonable person can conclude that it ought not to exist.
So I say let the money laundering commence. Or people will leave. Or they wont be allowed to.