Sure. But for South Korea and Japan it's less so; those nations already take a national security posture that leaves them exposed if the US were to turn against them.

South Korea and Japan have been allies of the US for quite a long time and have zero reason to think that would ever change.

There are lots of reason for a company or a country to have alternative independent supply chain (hardware or software or tech or anything).

Japan has to sign the Plaza Accord https://en.wikipedia.org/wiki/Plaza_Accord#Effects when it positioned to challenge US economy. It is attributed as the main reason to their Lost Decades. You bet that Japanese forgot about this, and Korean and Chinese do not know this.

What US gov initially wanted probably is something like Plaza Accord with Chinese, but they have the size to resist, so Trump showed the nuclear option. Like some commenters pointed out here, it may work in short term, but it will hurt US companies in long term.

Exactly - so "the US could flip a switch and sabotage our technology" may not be a particular concern for them.