USSR broke up because of external powers and and was accelerated by weak leadership. The same forces were trying to do the same to China but have all but given up.Sorry for the interruption but i see some methodological mistakes that have to be pointed out. You talk about ancient China as something united that reflects to the modern PRC almost identical, which is absolutely wrong. Even nowdays "Xi calls for national rejuventation" which i think this is like the Stalin's diplomatic nationalist "motherland's" calling for WW2 (which motherland, the Soviet? The Rus? The Pan-slavic? The Atheist? The Christian? The Muslim? The Buddhist? The Tribal?). National rejuventation is a call to form a new national conscioussness, above and beyond the present ones. To make my point more clear, i will bring examples comparable to China qualitative and quantitive. USSR after decades of propagating and impying the union, the union broke up, several slavic nations, speaking the same or at least similar language, having common past etc became enemies, several wars between these nations occured and 30 years after the USSR's break up there are many tension hotspots in the region. The quantitive example is Europe. Both Latin and Slavic alphabet have their roots in Greek. Greek culture and philosophy set the foundations for the coming cultures etc. If we advocate that Europe is somewhat "Grecified" we can present a situation where Franks attacking the Byzantines, or Venetians is something internal, something that have to do about the damn Greeks. Which is not! Keep in mind that Min speaking territories and Jin speaking territories are as far apart (or more) as France to Italy or Turkey.
I think Chinese weakness was correlated to contact with European powers who were able to take advantage of weaknesses like no enemy China had encountered before.When China was weak, the nomads invade and build their own dynasties. Gradually the offspring of the nomads became farmers and sinicized. Again this is not something that China actively sought, but rather voluntarily taken by the invaders.
And which China was weak? The Han? The Tang? The Qing?The Mandarin Speaking? the Jin speaking?
The same question could be put like which Greeks were weak? The Macedonians, The Romans, The Byzantines? The Greek speaking, the Latin speaking etc. IF this was not something that China actively sought, why China built the biggest ever fortification on the planet, the Great Wall?
As for your question about Greece - no offense but I don't think they've been anything other than weak for the past 2,000 years.
I don't see how that would be a nationalist position. Confucius is Chinese, Genghis Khan is Mongolian, what's the controversy in that?There is something foul to this kind of narration. This narration gives the courage to the average Greek nationalist to claim that he is the direct child of Plato and has the glorious Greek DNA. While he is just the mixture of generations of wars and cultures that passed through the region he was born. This narration makes the average Jai Hind to claim the glorious past, hoping to erase his unfortunate present. I believe that you see what i struggle to tell, no offence
You yourself claimed "Greek culture and philosophy set the foundations for the coming cultures etc". That sounds like nationalism to me.
Overall I have no idea what you are trying to claim. That China isn't a real country?