The threat of unauthorized or accidental launch is real, but is hard to assess since it depends on details of the Russian and Chinese command and control systems and their launch policies.
Some experts believe the Russian command and control system is deteriorating.
In any event, it would be prudent for the US to take measures to reduce the threat of unauthorized and accidental launches from Russia and China. But deploying a national missile defense system would be an ineffective and counterproductive response, and better solutions exist. Limited defenses could not be expected to cope with accidental or unauthorized launches. Depending on where in the command and control system a problem occurs, an accidental or unauthorized launch could be relatively large. For accidental launches, there is no reason to believe that a small attack would be more probable than a large one.