I don't know why people single out China for this when the US has invaded, couped, bombed, and otherwise helped install right-wing military juntas globally. Where was self-determination for Vietnam, Cambodia, and Laos?
China's priority is national security, and given the track record of western liberal democracies it is not unreasonable. As I recall we had a civil war the last time states tried seceding.
I’m not American, I don’t agree with the Vietnam war and I think the civil war was an ethical difficult decision. That was 150 years ago so not really sure why it is relevant.
Same for Catalonia, Hong Kong or any other.
Not saying it is a thing that should be done lightly, but also not impossible.
Countries exists for the people, not the other way round like China likes to think.