Given an ultimatum by whom? And is the ultimatum issuer OK with the other side of the coin? (That China will likely invade Taiwan the moment there's no global economic incentive not to?)

Global politics are always very messy and ultimatums don't help.

It is not like they are being kidnapped/brainwashed to work there aka Soviet Union style, or their genocidal war crimes are ignored like the US did with Nazi scientists.

Chinese companies are simply offering a good financial package to these people with valuable skill sets. TSMC can pay it's employees to retain they can certainly afford to.

Taiwan is not officially exporting talent to China.

No country wants their best people to leave, US and countries have used their wealth to attract the best from developing countries and benefited immensely, china is also using a similar strategy