There is zero US military - or any other military threat related to Taiwan or Hong Kong.
While there might be some possibility of US defence of Taiwan in the event of a Chinese invasion, there is no material threat to Chinese territorial integrity otherwise - from anybody.
There's no airbases, no missiles, no naval base, no ballistic missile launch points for US forces on Taiwan.
There is zero military justification for China to invade Hong Kong or Taiwan.
The capture of Taiwan and Hong Kong is 100% a Nationalist Imperialist objective of Han Supremacy and their version of identity, history etc.
Militarily, the invasion of Taiwan is going to be extremely expensive, hugely disruptive, and will risk giant swaths of Chinese trade, their international standing and participation in so many things. In fact - it already is. China faces a great deal of pushback for their moves on both HK and Taiwan.
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"I cannot imagine anyone who has seen a world map seriously advancing the idea of HK independence,"
There is a 100% chance, that if a choice were given to Hong Kong citizens today to either continue on the path to integrating with China - or - to be independent - they would chose independence.
Why don't we find out and give them the choice?
Hong Kong would dump China the instant they had the opportunity.
If the CCP had not moved towards controlling the political apparatus, Hong Kong would have been independent in 1997 whereupon China used the threat of violence to stop anyone from considering otherwise, as they do now: violence as the only instrument of power.
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"way ahead? On what metrics?"
Hong Kong and Taiwan are ahead of the mainland on all metrics.
2020 GDP/capita in USD:
HK $46K
Taiwan $33K
China $10K
In terms of quality of life, education, healthcare, political freedoms - Hong Kong and Taiwan provide immensely better for their citizens than China ever will.
The capture of Hong Kong by China is already resulting in it's decline, the same would happen to Taiwan.
Hong Kong serves as an important financial gateway for China into the 'real world' - the only reason that financial opportunity exists is because the historical independence of Hong Kong.
No more independence - no more special financial status - and so both China and Hong Kong lose.