if China invades Taiwan, there won't be much of TSMC left standing by the end.
What is the incentive? If China invades, anyone involved in destroying that asset will have a huge problem for themselves afterwards, China won't forgive. On the other hand, see what happened in Ukraine, where in the initial invasion there was quite a bit of collaboration of some Ukrainians, for example Antonov leadership ("Antonov's leadership sabotaged defense of Hostomel airport" -- https://english.nv.ua/nation/antonov-s-leadership-sabotaged-...).
If they will have to live under China's rule, the incentive for the individuals is to cooperate with them and be richly rewarded for it. Who wants to sacrifice themselves - and likely their families too - to benefit some outsiders? Who are leaving you behind to fight for yourself? Which at that point is against your own interest if you don't think you can win. Especially when the invaders are from one's own greater "tribe", so that it is not as bad as being invaded by another people (like the French or the Americans in Vietnam).
I did not forget this. I think you overestimate what the US can and is willing to do there, and also what the incentives would be for people to escape to the US. This is not Vietnam in the 1970s, staying right there may very well be the preferred choice, even if China takes over. Especially if they make a very nice offer in advance. Should Trump win that may look even worse.
The Americans will want the experience of the operators. If those operators refuse to go to America, they'll be unalived with the plant itself.