China's fear is being surrounded. Russia, even with its antiquated military, can put military pressure on China and distract China from its core interest which is the Far Eastern region. If Russia will eventually unite with Europe and become its own independent pole, it's a problem because a Russia with EU economic power backing it can actually produce modern hardware in numbers.
For Russia, its only true strategic fear should be from the East Asia region. Russia has had traditionally used its Russian winter to protect itself, but guess how cold Harbin is? Sure, Russia is colder, but it's not severely colder than the northernmost parts of China, and moreover, Russia has a vast and undeveloped backcountry. Guess who's facing this backcountry? No, not the Europeans, it's the East Asians.
So, I mean, I expect the Sino-Russian alliance to break down eventually, but hopefully not for decades. By then, Chinese defenses against the United States should already have been moved into Taiwan and Japan (i.e, China reunifies with Taiwan and Japan is part of the Chinese sphere of influence), and China should have expanded massively enough that if the Russians want to put troops on the border, the Chinese can easily afford to put tanks and artillery on the border as well.