USSR dissolved because of atrocious economy. It was hard for people to have their families fed, fixed prices and influx of money in the economy had led to a coupon system - basically a right to buy a certain quantity of sugar/dairy products/meat/etc. (I was born in the USSR and had lived through the agony of the regime)
Unless CCP somehow screws up the economy really BAD, driving everyone to the point of starvation, nothing will change.
And even then: looking at North Korea, people there were driven far beyond the point of starvation, and they still endure these hardships without any discontent against Kims.
I don't agree with your assessment of those events. 3 letter services had very little control over the process of dissolution, and their attempt to stop it (the GKChP putsch) was met with decisive resistance.
Of course, in most republics local elites have cleverly kept the reigns of power in their hands, but those were the Party officials, not KGB officers. I think that Russia is the only ex-USSR country where KGB managed to seize complete control. Others are democracies (Baltic countries and, arguably, Georgia and Ukraine, maybe even Armenia), authoritarian states (Belarus, Russia) and a number of sultanates.
Agreed with all your points but I don't think DPRK is a good comparison to PRC. DPRK is small enough to brainwash all its citizens and imprison those that refuse, not so for PRC (although they do seem to be trying this in Xinjiang).
I don't think small nation is easier to control. For example, in Russia, whoever controls Moscow controls the country (don't want to explain here why exactly, so please just trust me in this). So, theoretically, a moderately big popular uprising against Putin in Moscow could gather 300K-500K protesters, and Moscow police would have just 30K riot troops to deal with it, plus 40K regular policemen. However, in a span of few hour, reinforcements from all over the country can be brought in from all over the country, bringing the number of riot police to 150K, quite comfortably enough to counter 500K protesters. And regional protesters have no real way to join the protests in the epicenter, because Russia is freaking big, travel is controlled by the government and can be ceased in case of emergency.
Side note: 2013-2014 protests in the Ukraine worked for a simple reason - not only protests enjoyed an overwhelming support in the capital, and had a fraction of parliament supporting it, Kiev is relatively close to western Ukraine, so many active protesters from there just got on buses and drove to join the fight and make the balance more even.
This said, I believe that the bigger the country, the easier it is to control, because government forces are way more mobile, which easier brings local numerical superiority. As for imprisonment, in a bigger country you just need more prisons, that can be achieved without much difficulties.
