I've wondered about that as well. In fact, it's entirely possible that US could nuke itself if there is a civil conflict. However, I think the big danger US poses currently is that it's a single state in control of a massive nuclear arsenal. Should US break up, then this arsenal would be partitioned along with the rest of the military assets. I think that's a much better scenario because it would be highly unlikely that all the states would agree on coordinated action.

All that said, I completely agree that China having air defence that can take out the nuke before they hit would be a great thing. It is a very difficult problem however as we currently see in Ukraine. Russia has top notch air defence, yet things get through now and then. If we're talking about nukes then even missing one could mean millions of deaths.