Russia's population is dwindling, and over half of their country is Asian. China could take over Siberia in the future.
Darn, man. That's the cold war thinking. That's been over since the Soviet Union folded. What's left, is Mother Russia. And Mother Russia has the longest continuous land border of *any* nation on this planet. They span Europe and Asia. It is also *the* largest country in the world. It also has a relatively small population - 142M, about 1/3 of the US. It borders China, with population of 1.6B. It also contains the Urals and Siberia regions, probably some of the most mineral wealthy regions of the planet, as well. Now, ya think, perhaps, they'd be worried about the ChiComs getting ideas ? Not to mention, during the cold war, especially after Nixon went to China, Soviets and Chinese had *division* size engagements in border areas ? Meaning, shooting war.
Anyway, with Russian conventional forces a shadow of their former self, Russian mainland is pretty much indefensible. Only thing that's stopping the Chinese from getting a bit more "lebensraum", to quote their ideological cousin, is the fact that there wouldn't be about 500M Chinese after that little adventure. We're not a threat at all from that pespective - don't think anyone around here wants to go live in Siberia. But there are plenty of Chinese who would.