Couldn't the web be controlled from a legal standpoint? What if people had to fear imprisonment for downloading classified documents? I'm not asking this rhetorically; I'm hoping someone can point out why I'm incorrect.
The fundamental architecture could be changed to the point where it actually was controllable, for example if all packets had to be encrypted with a key that was provided to the sender by a centralised authority. This would make all the unencrypted traffic stand out like a sore thumb, and the encrypted traffic would be amenable to deep packet inspection and the like by the centralised authority. Banning encryption that does not use an escrow key would of course also be necessary, but this would also stand out like a sore thumb.
To say the technical challenges of actually implementing the aforementioned approach are non trivial however is to put it mildly, not to mention the necessary political hurdles and the degree of power the centralised authority in question would need to have to practically pull such a plan off. We're not there by a long shot, and it appears that the majority of people who want us to get there do not have the intellectual capital to pull it off or indeed even come up with it, or indeed even understand the reason that they would need to come up with it.
And if you're watching, you can't steal mine, I've patented it. ;)
The dangerous thing about containing dissident ideas is that when a society begins to function less well, the average person has greater incentive to learn about alternative ideas. So alternative ideas become readily available exactly when they seem most plausible. That's what has historically made authoritarian regimes fragile.