What is the operational support infrastructure in APAC region for these submarines if China understands what's coming & destroys supporting bases in the region?
I think your response misses my point. You are focusing on the practical difficulty of deploying and sustaining large numbers of US submarines in the Pacific theater.
What I am arguing against is the assumption that “USN boats are spread globally” in a meaningful strategic sense during an existential US–China war.
In that kind of conflict, the US would obviously attempt to concentrate as many deployable submarines as possible into the decisive theater. The limiting factors may be maintenance cycles, transit times, logistics, operational support, or survivability of regional bases — but not peacetime-style commitments to other regions.
The same logic applies across the broader US military as well. In an existential conflict, the USAF and other branches would also prioritize concentrating combat power into the primary theater rather than maintaining normal global posture.
So from a Chinese war-planning perspective, it would be a mistake to assume the US would keep its forces globally dispersed during such a war. The safer assumption is that the US would surge the overwhelming majority of its deployable combat power into the Pacific.