But the great firewall of China works so well. Yes, it's trivially circumvented by anyone who knows how things works, but the vast majority of Chinese people don't fall in that category.

The purpose of these systems is to keep the general population in the dark, it's not targeted at knowledgeable intellectuals. You can't keep them in the dark anyway and they already know. It's only point is to stop knowledge dissipating to the masses.