It's a point worth making that Bitcoin doesn't have a single central hub that can be killed easily.

Things that can't be shut down centrally may continue indefinitely under crackdown, but their scope will be drastically reduced, and they will be done only by a handful of very seedy and/or reckless individuals. Even mild and sporadic enforcement (such as e.g. that practiced against drug dealing) is enough to make the activity out of bounds for respectable people and legal businesses. Really severe enforcement would make even the seediest people think twice.

Thorough and effective law enforcement is an easy problem for modern governments as long as there is the political will to do what is necessary. This is often not the case for a variety of reasons, but you can bet there will be plenty of political will if Bitcoin ever becomes subversive on a large scale.