1. Bitcoin transactions don’t require access to the internet at the time the transaction takes place. There have been solutions for performing offline transactions, though none have really taken off. Nevertheless, it’s not hard to do, though it’s riskier for the recipient than an online transaction. I won’t go into the technical details, but it’s nothing fancy. The idea is that you can place money in any number of accounts ahead of time, then simply pay people by handing them the keys to those accounts. There’s little overhead for creating new accounts; often merchants will use a fresh account for every transaction. Of course, the recipient has to be able to trust that they’re actually receiving keys to an account with the right amount of currency, which is a harder problem to solve.
2. This isn’t necessary. Bitcoin was popular in certain criminal communities long before there was any easy way to convert between Bitcoin and fiat. Cashing out dirty Bitcoin to USD remains risky.
3. The UX is already terrible. The UX that criminals experience isn’t the same UX that investors experience. They can’t use services like Coinbase to cash out.
4. Prior to Bitcoin going mainstream, the cover was that you could exchange your Bitcoin for various illegal products and services. (Want to buy a stolen car?) When there’s a whole underground economy whose participants are thoroughly convinced they are outside the reach of the law, any currency will work, as long as there’s a consensus within that community.
Banning cryptocurrency would have some interesting effects, but it would be nearly impossible to enforce, and it wouldn’t have the impact you’re seeking.