Democracy is the most powerful weapon:
I personally think China will surely democratize smoothly in the next few decades. There really won't be a defining moment; it will be a gradual process. The Chinese authoritarian state is/will gradually morph(ing) into a one-party state with increasingly full civil liberties. Eventually it will look like Japan for the last 40 years, where the dominant political party held control but politicians competed for control of that party. Opposition parties may compete, but will generally do poorly. Finally one day, an opposition party will win an election, but hardly anyone will notice, as happened in Japan last year.