Japan would be weaker in an alliance with the PRC because the PRC seeks what would weaken Japan strategic situation with claim on Senkaku islands and Taiwan. Also, when Japan past the new defense bills, the US, Australia, the Philippines, India, etc. supported Japan. It was the PRC warning Japan to "not forget history" in indication to troll history narrative and criticized Japan in passing the new defense laws. PRC is criticizing Japan for considering preemptive capacity as well. So very obviously, the PRC aims for a weaker Japan than what the US finds acceptable. And you know it becausewe debated it many times and each time you ignore it.