I don't think the US striking the PRC mainland, at least outside of an immediate conflict zone like the shores of the Taiwan straight, is plausible either. It's

I don't think the US striking the PRC mainland, at least outside of an immediate conflict zone like the shores of the Taiwan straight, is plausible either. It's exactly the same arithmetic. Why would the US or China reduce their readiness to respond to an attack because their adversary has become more capable? The whole point of a nuclear deterrence is to deter attack. If anything, making it clear that a large scale missile attack will be interpreted as a nuclear first strike is more beneficial when there are a bunch of missiles the adversary is seriously considering launching at you. It is in both countries best interests to keep any potential conflict limited with clearly delineated theaters of war.