Taiwan and South Korea are hardly our geopolitical rivals.

Oh, come on. We both know that in a war, our main geopolitical rival would conquer both on turn 1.

We really don't. Naval invasions are hard, particularly when the US is the world's dominant sea power. Theoretically China could invade South Korea through North Korea but North Korea wouldn't like that and they have nukes. But conquest of them is what happens when WWIII cooks off - we were talking about geopolitical rivalry.

Even if China can't invade Taiwan right away, China can set up a naval and air blockade. And if they can't do that, they can destroy the very delicate fabs using short-range conventional missiles. China is a high-tech peer adversary, not a bunch of desert guys in pickup trucks. As a matter of national security, we need a secure (and E2E-trusted!) supply chain for advanced technology.