Well, you probably do not know the concept of MAD and nuclear winter. As long as the US is cripple, it will not just sit there. It will make sure to annihilate all other industrialized areas, in order to make sure that the US does not fall victim to a future where she herself is economically crippled, while another industrial nation is entirely intact.
Imagine the scenario where US is crippled and China is annihilated and the rest of the world is untouched. The US will end becoming a prey to those untouched industrial nations: Europe, Russia and Japan. In a way, all China need is the ability to ensure that she can penetrate the US missile defense in a second strike and destroy enough of the US economy to trigger MAD and Nuclear Winter.