The point is if China decide to do this, there isn't anyone who can stop them.
After it became part of China again, it is not possible to interviene militarily in Beijing - HK relationship - that would be an invasion of China and violation of international law. You could not phisically station an army in HK.
Even if China has violated an agreement with UK regarding HK autonomy, thay does not give you legal grounds to start a war.
In cotrast, you could deploy the entire US military in Taiwan tomorrow, it would be legal and peacefull (provided Taiwanese Gov. Invited it). Additionally, invading Taiwan is actually extremely difficult because of terrain, just because you have a massive land army does not mean they can just teleport to Taiwan. It would be basically a D-day style effort against a well entrenched opponent.
So it is unlikely we will see phisical takeover of Taiwan in the near future, as so far we have no indication that China is looming for a causualty heavy war with little gain.
Taiwan's history is complicated. it have its own military. its people pay taxes to Taiwan, not China. Taiwan's safety is its own responsibility. its more than possible that China can take Taiwan by force but at what price. its not like Taiwan's military have no teeth.
This is true, but I view China's economic growth more as 'reversion to the mean' when their population is 3x that of the US.
Is their military power proven and able to assert power or enforce its will?
Will America maintain its might?
See Cuba and the Soviet Union