I'm not sure why you say this. Germany and Italy didn't collapse-- they were conquered. The Soviet Union did collapse, it's true, but it collapsed because of external pressures and sclerotic bureaucracy causing an inability to adjust to internal problems. In all three cases there was an external force contributing to the problem. However, there's no reason to assume that China's authoritarian rule will collapse as well. Indeed, in several Soviet successor states (including Russia), authoritarianism has reasserted itself.

Authoritarianism is easy to impose and democracy is fragile; the civic discourse required for a strong and stable democracy relies upon a considerable amount of mutual respect and rhetorical restraint.