Needless to say that its also technically impossible to ban crypto. Its like banning torrents or even harder and we all know torrent was attacked heavily in the past but it did not go away.
No one wants to ban crypto. The goal is to ban financial institutions that handle fiat from trading with institutions that handle crypto.
From there, if people want to accept funny money as payment for pizzas, it's up to them really.
All black markets have an incentive to use it - including state actors (e.g. weapons deals). Getting caught laundering Monero is preferable to getting caught selling missiles to the Saudis.
The hardcore crypto types would carry on using it but like 95% would give up - that's sufficient.
Another use is tax evasion and money laundering.
Why is crypto any better than cash if users are forced to deal in person? I understand the argument of not having to carry around a duffel bag that could be seized. But nations can disrupt crypto markets on a global scale.
At the very least, they can make it difficult to exchange for actual, usable, local currency. Which appears to be what China is poised to do.
Odds are that, given fiat to launder, or crypto that needs to be converted to fiat that still needs to be laundered, they would cut the useless step.
The only illegal industry that would still see the use for it is maybe ransomware, but ransomware is not a cyclic business, so it would be surprising that they'd find enough individuals on the street to convert their millions easily.
Banning crypto in a practical sense is actually quite easy. Prohibit any business and any bank in dealing with them. If you cannot do legal transactions with e.g. Bitcoins any more, the market mostly dries up. It is not necessary to ban any single coin in existance, but its wide-scale usage in the normal economy.
By the way, there was never an attempt to ban torrents itself. That would have been reasonably easy on a protocol level. It was about banning illegal torrents. As in the content, not the protocol.