EU rearming is not so bad for China because they can't afford to rearm both navy and army/air force simultaneously. Navy is not so useful against Russia, Russian SSNs own the Arctic and they have to worry about being overrun on land if they're not careful. So they invest in army and air force which is not so useful against China.I don't think this is the case in the long term.
1) Europe will rearm for sure and be fully aligned with the U.S.
2) Belt and Road is probably dead in Europe, hopefully they can salvage some of it but I don't see how.
3) Performance of Russia's conventional military hasn't been the best, it would degrade European and U.S. perception of their capabilities. Which would in turn lead to
4) U.S. will be able to focus more on the Indo-Pacific.
Comparison with 9/11 is not appropriate since Al-Queda and Taliba were NOT a part of the Chinese camp, but the Russians are.
Increased military spending also means they spend less on what really counts against China: industrial modernization and social stability. China is surpassing every EU country in software, electronics, telecom, space and semiconductor, and on their heels even in traditional machine building. They need to catch up, but can't because they rely on all those sectors on the US. Then Russia is still #3 in space far above ESA, they still have significant machinery capability, and they have the energy trump card.
Speaking of which, now EU also has a doubling or so of energy costs if Russia cuts them off, making their industrial upgrade even harder. This weighs on them even more.