It's true China cannot absolutely filter the web. However the majority of the people won't bother with VPN. China doesn't need to make dissident ideas absolutely contained to be effective. They just need to make it sufficiently hard for those ideas to spread.
It depends on how strongly the ideas resonate with the populace.
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.
It's not about the idea. It's the execution. China has the power to effectively mold the minds of the majority (like 80-90%), and the majority is all that's needed. The idea may be in the back of the minds of a lot of people but China can prevent them from coordinating, spreading, and developing those ideas.
If China had a popular democracy, they'd only need to mold the minds of 51% of the populace (assuming Australian-style compulsory voting, or 31% assuming US-et-al-style voluntary voting).
Everything gets harder as things change and you need to adapt. But I don't think China is having too much trouble adapting. Their tactics are working at shaping the population's minds. They may not be able to bend minds but they certainly can mold and guide it.
