Private bitcoin becomes useless if outlawed (assuming most people do not want to live in a sort of Christiania). To outlaw private bitcoin you just need a law in your jurisdiction. Then you will have to decide to KYC and live in society (and pay whatever taxes your are forced to pay) or become at outlaw and take your private bitcoins to your bitcoin island. The fact that gov can’t ban bitcoin technically does not mean at all it cannot make it useless taxing the hell out of most people (just as an example) At least for the 90% of people that do not appreciate having to become a pariah to buy a beer with private BTC.