Do you think Russian leaders should allow this to happen?
Democratisation through osmosis has utterly failed. The result has been the subjugation of Hong Kong, genocide in Xinjiang and the invasion of Ukraine. Taiwan is next unless we wake up and make it perfectly clear to these people we absolutely will meet force with force and that we have plenty of hard power to use alongside our soft power. It may be too late for Ukraine, we failed them miserably by playing nice too long, but it's not too late for Taiwan.
The fact that Taiwan has been a us ally for decades and the US has strongly implied in the past and now clearly stated that they will defend them matters. The US had never even implied they would defend Ukraine and had no treaty obligation to do so.
Also China is not Russia, while it's building a formidable navy, air-force and ground force and may be more capable than their Russian conventional counterparts, their nuclear deterrent may not be enough to deter an American intervention in Taiwan. Their nuclear force is small, and though rapidly growing, currently not enough to reliably threaten the US, the destruction is not so mutually assured in the China US equation.