I don't really agree with your basic conditions - 'Japan' has existed in pretty much its current state for most of history,
Yeah, Japan's an exception.
'China' has shifted around but been a reasonably coherent entity since several thousand BC -
That's definitely not the way I learned it. The was even a century where splintered Chinese states would rise and fall by the decade. Maybe they all referred to themselves as 'Chinese', but we don't accept that Byzantium is the same entity as Rome because they declared themselves Roman.
meanwhile in the west I don't see the Roman Empire around,
I meant post-Rome, in the early Middle Ages.
and the borders of nearly all states have changed massively. Spain dates to the Middle Ages (and then only with difficulty),
I'm not talking about borders. Castille existed for quite some time, and absorbed other states into it. But it was never replaced by some other polity. Let's look at Iran, for contrast. It almost united itself under the Medes, succeeded in doing so with Cyrus, whose empire was conquered and replaced by the Macedonians, who fell to the Parthians of Central Asia, who were themselves removed by the native Sassanians. Arabs, Saffarids, Samanids, Buyids, Seljuks, Ghaznavids, Khwarazmians, Mongols and the Ilkhanate, Timurids, Safavids, and Ashfarids all followed until modern Iran takes shape. These were not just different family names ruling over the same countries, as in Europe. Each dynasty to an extent
was its own empire, with a different power base, ethnicity, ruling class, and often religion. All of them had borders which fluctuated wildly; even Rome's borders remained stable long after completing its expansion. It's more extreme than if an equivalent of the Norman conquest happened to England every couple centuries.
Italy to the 1860s, Germany to the 1870s. 'England' has been around for a long time, but the territory controlled by the people who control it has only been stable since the 1920s. Belgium is a product of 1815... there are many examples.
I don't think that modern nationalism really counts. It spread to the rest of the world within a century of your time frame and by its nature fixes borders between ethnic groups.
I think a European history class is in order... Have you ever heard of the Holy Roman Empire? (Just one example containing hundreds of non-stable political entities.)
Plenty of those also lasted for centuries and some formed the precursors to modern states and provinces. I don't imagine Luxembourg would exist as an independent state outside of Western Europe.
Another thing: with very few exceptions (Crusades, Russia) the migratory pattern in Eurasia for the past millennium was been east to west. I think this is very illuminating for the topic, but I'm not sure why it happened.