If ever it is banned, I think underground transactions and groups would flourish and still continue what was started 7 years ago. Underground marketplaces where people still asks for bitcoin for their goods including illegal merchandise and whatnot. it would still be there, the thing is there would be no third-party legal services that would cater exchanging bitcoin into fiat, same goes for registered merchants on and off the web.
For sure if bitcoin would be banned completely then there would be probably underground marketplace and hidden transactions would be created and we really cant stop it because people would really find ways in able to transact no matter what.
Underground market is the way to go for bitcoin if it is banned, so yeah I agree with what you said. Though being underground, you will see a noticeable price increase because it has an additional risk charge for the transaction. People especially those who are involved in illegal activities will be the one to continue in using this coin. I will stop using bitcoin if it is banned.