Think they can, and Napster/Kazaa shutdowns (PirateBay) are the blueprint. You TECHNICALLY can't shut crypto down, but you can make it illegal activity like the US outlawed gold ownership from 1933-1972. This puts 95-99% of people "in line".
The rest are (dark side of the force) outlaws, and run the risk of jailtime. So yes, a dark crypto will eventually become the backbone of black markets, but how many people can afford to run the imprisonment risk?
We agree, you can't shut crypto down, but you can deter the masses from using it rather easily, and they already are starting (with the SEC crackdown on "semi-legal" exchanges). But they certainly clever in how the subterfuge it, ey?