People are going to laugh at this, but it seems to me that Russia should be more concerned about China. China wants oil, timber, and farm land and what country has it all right next door, and has no allies who would come to its aid? Last century the USSR and Nazi Germany were sort-of-allies for a while, then Germany decided it wanted land and resources from the USSR and +30 million died in the next few years sorting that out.
Sure, the Russians are far better soldiers and would slaughter a lot of Chinese in the process, but what does China care? China has 10X the population as Russia, and China might actually be interested in the prospect of getting rid of a hundred million excess peasants in the process.
If China invades the Republic of China, the humaniatarian crisis on display in that densely populated country could push most of the world into containing China, crashing the Chinese economy and resulting in revolution. But if China contrives an excuse to invade Siberia, who would complain about it? In sparsely populated areas, the casualties are soliders, not civilians, and people care less for soldiers than for civilians. "Russia is always invading a neighbor, so now it is their turn--what goes around comes around." There were border skirmishes between the two countries in the 1960's if memory serves. It seems the only thing keeping the Chinese from rolling over half of Russia is the prospect of nuclear warheads landing in Beijing. The calculation may be how much land Russia would be willing to lose before triggering a mutually-assured-destruction nuclear exchange with China.
That's a terribly archaic view on international politics and practical reality.
1. Sitrep on resources. Fact is Russia actually doesn't have a lot of arable land in Siberia, most of it is frosty tundra, steppe and pine forests. For the good farmland you'll have to go to the southwestern parts of Russia, say Voronezh, and Ukraine, which is obviously impractical for China to reach. As for oil, most of it is either in the Arctic seas or near the Urals - both of which are too far for China to be profitable. Other than that and some mines, Siberia is pretty much a wasteland, I don't think the US would want to invade Canada for Yukon even if it's a resource-starved nation, no?
2. Cost-effectiveness. It isn't like Russia isn't selling them to China, and for a fairly good price too. Your view would be quite spot on if it's 1850, but not 2021. Owning the land itself on which non-rare natural resources situate has become more and more unprofitable ever since 1900. Germany was never interested in owning Russian land and resources until Babarossa after which cutting off oil for the Soviets suddenly became important. And you've probably never been in Russia, because if you've been there you'll know Russia is a bit of a third world country outside Moscow and St. Petersburg. If China conquered Russia, it would have to spend too much to properly occupy Siberia to prevent it to be returnable ever ie. colonise Siberia with say 5mil people, build roads, railroads, maintain military outposts and supply lines for like 500,000 mechanised troops. This means it simply isn't worth it when the Russians are happy to sell in the beginning. Plus the Chinese don't have any spare population to spend. Ever since the Industrial Age countries have been trying to boost their population numbers because every new mouth is potentially more labour to produce and products to be sold which keeps the economy afloat. The US economy boomed since the late 1880s because it had a bigger population than Europe and it kept the economy growing at a high speed continuously simply because there's more business to be had compared to that across the pond.
3. International politics. China and Russia are in a de facto alliance against the (((US and other countries))), which means they will never backstab each other even if they don't see eye to eye on everything. In fact, allies shouldn't ever have to see eye to eye on everything, the current world order is a system of manipulation by (((the same perpetrators))), and in a truly independent world, independent countries are entitled to have their own views on issues according to their own interests. Having differences doesn't mean they'll go to war.
4. Combat capabilities. Obviously you still have the 1980s mentality thinking the Russian army is some sort of steamrolling machine. It's not the beast it was anymore. Take its army for example. Russia has an undermanned, underequipped and unmodernised army due to its poor economical situation, so much so it has to reorganize its army into Battalion Tactical Groups - every regiment focuses on their efforts and training on just 1 rapid reaction battalion known as a BTG, and the rest of the regiment is assigned to reserve duty. What this means is that 1 regiment basically becomes ineffective if its 1000-man strong battalion is destroyed because it doesn't have to equipment to rearm its reserves. Other than that Russian are sorely lacking in modern equipment. For example Russia doesn't even have any drones in its reconnaissance company or its air force for that matter. That's mainly because Russia has never been good at electronics/avionics and it has always been struggling with cutting off enough weight for its planes to fly. This is why Russia hasn't been able to make its own 5th gen fighter until last year, 22 years after the US and 10 years behind China. The last time Russia made its own functioning surface combatant it was a 3,500 ton frigate, which speaks volumes about Russia's arms industry. The Russian army of today may still be able to thrash Ukraine and the lazy government paper-pusher armies of Europe, but it is definitely not up to the task to deal with a "rich" army like China's and the US. It's a nuke-wielding Saudi Arabia.