If you look at transitions like the english civil war, or the french revolution, the clear precondition of revolution is not 'stability', as you think, but rather systemic dysfunction compounded by incompetent leadership. This kind of situation is one which democracies, in theory, should be much less vulnerable to.
Your ideas about the relative balance of power between state and people make some sense, I think, but they only matter when the threat is internal dissent. Most of the states in modern europe had a form of republicanism enforced on them by the French, who were able to invade all their neighbors because their republican government was (while very dysfunctional) more efficient and able to field talented officers and large armies.
A modernised military, moreover, consists of normal people - and they will also feel the disillusionment and apathy that grips really dysfunctional regimes. Saudi Arabia, for example, fields armies of terrible soldiers, using the most advanced weapons available.
China is an interesting and very weird state, because it's a very old civilization with deep roots, with very different basic ideas to the west, and they have turned marxism into a kind of managerial culture for an extremely capitalist society. I don't know if they will follow any of the typical patterns that totalitarian states follow. You can't really use the USSR as a point of comparison, because the USSR was a very European project, coming directly out of the enlightenment, and the western political tradition. Nor would Korea make sense, since they were colonized, and both North and South represent different reactions to colonial subjugation.