Who says? The best defense is a good offense. It should be clear to everyone here that China dominates the FIC and that dominance will only become more lopsided as time goes on. The PLAN should not and will not be committed to "AD" against US naval forces, it will be committed to destroying US naval forces.
Yes, on Japan.
China is not going to tickle Japan with a few ballistics missiles. In a war, it would pummel Japan's assets in a PLARF initial strike, destroy whatever remains with PLAAF and PLAN attacks, and then blockade and bomb Japan until it surrenders.
Why not? What are Japan's air defenses? The same systems that so successfully defended Abqaiq against some Houthi drones?
The West has significant fire inferiority in the FIC. Any war restricted to that theatre will very likely result in a decisive Chinese victory.
China's problem is to project power beyond the FIC and into the central Pacific/Indian Ocean so its adversaries cannot escalate a conflict by attempting to blockade China's SLOCs. That and hardening its economy to eliminate dependencies on foreign technology and energy. Happily, there is excellent progress on all these fronts.