Easier said than done.
With crypto, you can move a billion dollars in 15 minutes to anywhere in the world for like $15, without giving away either source or destination. To do this, you need to send about 250 bytes of information to any node in the blockchain network, using any way possible, up to and including dictating these numbers by phone or writing them on a piece of paper and smuggling said piece to anybody with a non-censored internet connection.
Goverments can make this illegal, but it is impossible to enforce it. These 250 bytes are just another "illegal number", which can be written anywhere, sewn on your t-shirt, etched in stone, etc. There are many fully secure ways to transmit 250 bytes without anyone knowing about it (including governments). Therefore, there is a secure and bulletproof way to smuggle a billion dollars out of the country without getting stopped.
No one can do anything about it.
(Well, there is a crypto-sanctions mechanism which can taint some money on the blockchain and can make it difficult to sell on exchanges etc. But it just creates some inconvenience -- there are many ways to launder crypto, from mixers to bridges and coinjoin and anonymizing through fees arbitrage and whatever).
> No one can do anything about it
Maybe online, but with state actors the fear is what they can/will do to you IRL and how to avoid ever raising suspicion. And moving a billion dollars overnight is something that I think is impossible to do with raising eyebrows, regardless of the technical mechanism to move it.
If you observe the activity on blockchains, there are occasionally billion dollar-class transfers visible. Nobody knows who is behind them.
if you make it illegal to convert btc to cash (which a government can do trivially), my contention is that this will kill crypto. Most people simply don't want to break the law. Much of the liquidity in the system dries up. Yes you can break the law if you want, but that just goes to show that crypto's number one real world use case is to skirt regulations.
Regulations are not always a force of good to be observed all the time, particularly in non-democratic countries.