Even if Russia eventually "wins" the Ukraine War Russia's economic and military power will have declined compared to China’s.
China is not in political, economic or military alliance with Russia. Their relationship is centred more on self-interest than shared values. They are united though in an attitude that the US is the main enemy.
Russia’s inability to quickly beat Ukraine has lowered Russian morale and demonstrated to China that Russia’s conventional military is not as formidable as previously assumed. Russia’s tanks were previously seen as high quality but this has been disproven by the rapidity of their destruction by merely shoulder fired Ukrainian missiles.
China resents some of the border arrangements coerced by Russia over a century ago. In 1858 (Treaty of Aigun) Russia annexed the land north of the Amur River and in 1860 (Treaty of Beijing) annexed Chinese coastal territory down to Vladivostok.
Military Balance
Major conventional military and nuclear enmity was underlined by the 1969 Sino-Soviet border conflict which almost escalated to a nuclear exchange.
In comparing China with Russia China has more than 5 times Russia's national GDP, more than 3 times Russia's military spending covering fewer threats than Russia's. Russia’s main advantage is in nuclear warheads, 6,257 compared to 350 in 2018.
China has embarked on a major drive to install more ICBMs in Western China which in the long term may translate to parity with Russia's warhead numbers.
The Russian defence-strategic budgets must balance nuclear and conventional military spending. The sharp rise in Russian conventional military spending and weapon losses in Ukraine mean less Russian defence spending available for Russian nuclear weapons. Russia’s nuclear weapon arsenal is a very high cost for the relatively small Russian economy.
Russia would be mindful that China is a rising nuclear competitor. Russia's nuclear weapons include Russian nuclear submarine (SSBN and SSGN) platforms, nuclear armed jet bombers, ICBMs and smaller tactical nukes. Over a standard 30 year life cycle Russian nuclear weapons and their platforms are not a static cost. Platforms and weapons need constant protection, maintenance, development and replacement to remain competitive.
Economic Decline
China, has more often been in economic competition with Russia than a subsidizer of the Russian economy. A prolonged Russian invasion of Ukraine, win or loose, would be a win-win situation for China. The economic and conventional military costs for Russia would fall in China’s favour in the short-medium and long term.
Russia is gradually losing its pre-war level of trade in gas and oil exports to West Europe and to Japan. This will place largely capitalist China in a stronger bargaining position over Russian gas and oil and the Russian economy generally.
The war is also causing a “brain drain” of middle class Russians leaving the country impacted by financial hardship, uncertainty and fear of being called up to fight in Ukraine. They may be taking money with them by such unofficial means as cryptocurrencies. Immigration from Russia has increased to places such as Finland and Bali - destinations with more stable economic conditions and a more predictable lifestyle. This Russian brain drain is making China comparatively stronger economically.
China's Comparative Power Growing
Fewer Russian economic levers and military resources are available on the China-Russia border and in contested central Asian ‘stans and Mongolian buffer zones.
This would put China in a stronger position to, at a minimum, press for better terms of trade, lower oil/gas prices, maybe even joint developments rights of Far Eastern Siberian oil/gas fields including those in the Sea of Okhotsk.
Longer term Russian economic and military declines may see increasing Chinese submarine, surface ship and airbase visiting rights to East Siberian Russian bases. This includes Russia's Pacific Fleet bases at Vladivostok (SSKs and ships) and more advantageously Russia’s northeast Siberian Rybachiy (nuclear submarine) bases. For the latter see "Across Avacha Bay from the city in Vilyuchinsk is Russia's largest submarine base, the Rybachiy Nuclear Submarine Base relies on icebreakers, established during the Soviet period and still used by the Russian Navy." Over the medium-long term global warming's depletion of Arctic ice may eventually make Rybachiy an ice free base. Importantly the Rybachiy Base would permit China’s navy to operate outside first island chain restrictions.
Chinese visiting rights to Russian bases may be upgraded to joint ownership in future in a similar way to the US and Japanese navies having ownership of Yokosuka Naval Base.
Russia high tech weapon exports to China may one day extend to trade in Russian nuclear submarine hull and reactor technology to improve China's nuclear submarine arm. This would improve China's power projection internationally, especially in the Indo-Pacific.
Russia's invasion of Ukraine is proving beneficial to China in ways unexpected. Even if Russia wins, the longer the war lasts the better for China.