there is *no such thing* as a good reason to nuke China, Russia
You can't seriously believe that. Admittedly, it was a bigger concern back when the Soviets had overwhelming conventional force which we could not match. If those were the circumstances today, I would be willing to risk a Soviet retaliatory strike to prevent all Portugal to Helsinki falling under totalitarism.
This is also why we must not only maintain nuclear superiority, but conventional superiority. I want the decision to begin the cataclysmic event or face an enormous military defeat to be Putin's, not Obama's. It takes very little relative resolve to stand up to China conventionally if they invaded Japan, but much more to risk nuclear. Also, this is why I support a Japanese nuclear arsenal.
For example, Russia invades Estonia and threatens nuclear war if we resist. We should indubitably resist, and not paralytically fear the bomb. That step becomes much easier if Russia believes we are willing to pre-empt it if it starts to behave like it will go through with the threat.
If we unilaterally went to a minimum deterrence posture, like naive idiots in the vein of Carl Sagan would suggest, we risk Putin calculating (likely correctly) that we won't risk resisting him because it means the death of nearly one of us.