If push comes to shove Beijing will kill the "not-so-golden-now goose". Especially now that Xi has effectively consolidated control over the coercive forces of the Chinese state. As long as the security and the army is with him it will work. Worse case the SAR region is dissolved and a party official is sent to govern directly. A lot of people said if Beijing cracked down in '89 that would be the end of the regime bu…
Nah, the worst case is that some of the protesters acquire nuclear weapons, threaten mainland China with them, Xi calls their bluff and moves in, it wasn't a bluff, Xi blames America, nuclear World War 3 starts.
Or perhaps this peaceful protest gets quashed, and a violent insurrection that lasts decades begins. Beijing has a lot of places to hide IEDs.
But it is possible that this dies down and Hong Kong peacefully submits to communism.