Your caveats seem to make the rest of the post a bit pointless, although it is very interesting! You say that dictatorships have not been stable up till now, i.e. in conditions we are used to. But the central xrisk concern about dictatorships is their stability in new conditions i.e. world government or advanced surveillance. The stability of dictatorships in the past or at present doesn't seem to have much bearing on their stability in the future.

Or rather, it seems to have bearing in the more limited sense that we can examine particular causes of past instability and consider dictatorship-stability without them. So for example isolation seems key. If the USSR was the sole state in the world, it would probably be around today. A major reason for its end was elite exposure to outside ideas, mass exposure to differences in living standards and the cost of competition with the West. If an isolated totalitarian state is able to keep a strong enough grip on its education, media and power structure to prevent people with alternative ideas from gaining power or influence, it seems like it could last a long time.

This seems to be the main problem with a singleton. I sometimes imagine the future as threading the needle between increased control and global coordination (to prevent nano- and bio-risks) and the possibility of dictatorship.