I think you misjudge the Chinese Communist Party. This isn’t anyone else’s business in their eyes, just like their building military bases all over the South China Sea isn’t anyone else’s business. It’s an internal affair, and HK is officially a part of China. China is the engine of world trade, they know any reaction will be temporary. Trump has already started rolling back on Huwai embargoes.
‘Law and order has broken down in HK, our territory. The People’s Liberation Army will intervene temporarily to restore order. Financial and goods trading will continue as normal”. End of one country two systems, the PLA will never leave, Direct rule from Beijing, international protests, no action. Important to remember that almost everybody who lives in HK is the descendant (only one or two generations) of someone who took the decision to get out of Communist China. They are all ideologically suspect to the CCP.
The protesters are running a big personal risk, but the endgame has been obvious since the treaty ceding sovereignty back to China was signed, it’s just a matter of when. I’m willing to bet thatThatcher and her diplomats thought that the Chinese Communist Party would be long gone before the 50 years are up, and that they’d struck a good deal. To be fair, if China wanted HK it could have just walked in and taken it, so any deal was better than a forcible annexation.
And having said all this the HK Chinese showed almost no interest in having more democratic institutions when the British colonial government tried to get it going (decades too late) in the mid 80s. At district elections where I managed a polling station turnout was less than 30%.
The difference I see in China now, is that China is much less insular and is looking for commercial relationships worldwide. I don't think they will take this for granted. Too heavy handed on HK, and years of PR work by Xi and his lieutenants will have been wasted. I expect to see a gradual tightening of law and order, not a repeat of Tianimum Square