The USA is also willing to defend Taiwan and currently the US has carrier groups in the south China sea. This suggests that the idea of building chips in the US may be a diplomatic courtesy and arguably an incentive.
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Personally, I think we're at the point where the US would consider the costs of Taiwan's defense well beyond any potential benefits. It's too economically integrated with China, and the sheer number of bodies it would take aren't worth the moral victory.
Hong Kong is the writing on the wall. China wishes to restore integrity to what it regards as its territory.
TSMC must open facilities in the west because its Taiwan facilities are too dangerous to leave in enemy hands. It has to invest in capital outside of any potential conflict zone if it plans to exists over the long-term as a profit-making entity.
This may not be likely today, but China has been forced to develop its chip industry in parallel for a long time, and they admit as much if you watch their state-sponsored english-language semiconductor media commentary on YouTube. Eventually they will not need Taiwan nearly as much as America does.
The only way to sidestep that problem is for the US to do the same as Taiwan: use government power and resources to establish a competitive semiconductor hub in the Americas.
Funnily enough, South Korea has done the same in all practical senses. Samsung makes up more of South Korea's GDP in percentage terms than TSMC's share of Taiwan’s GDP. Samsung's trade policy interests are essentially South Korea's trade policy interests.