The handover treaty did not promise "an erosion of civil liberties in 2047." What it promised was that the territory would retain its basic law, constitution and freedoms until at least 2047. What was to happen next was to be up to the CCP and HK. You seem to think that moving to authoritarian control of HK was a foregone conclusion in 2047, but the past and present have shown great benefit to both of retaining 1C2S. Domestic peace, and an sort of a no-mans land where the rest of the world can meet the (relatively) insular CCP and do business.

The future, indefinitely, could have been 1C2S. In theory, it could still be.

It's really the CCP that's choosing to impose its will on the people of Hong Kong -- to your point, prematurely, but also objectively as a belligerent in this conflict. I suspect that most of the people of Hong Kong would have happily continued to live their lives on the island as they have for 50+ years, an enclave with a different way of life.