I'm not so sure. .onion sites are already close to anonymous and uncensorable. the people who do illegal stuff on the darknet seem to get caught because of personal mistakes much more often than a vulnerability of the medium itself.

even if you had some perfectly anonymous, uncensorable medium with zero security flaws, people would still make mistakes and get caught. it's never impossible for the state to catch someone, but it can certainly be too expensive to be worth it. this is a feature imo, not a bug. it should be expensive to investigate people.