Under MAD, even a single person can start a nuclear war, just by using a WMD on us, ( or our allies).

So you are saying that if any WMD are used on us, whether or not they are delivered by a missile or by some other means, it will start a nuclear war. This is exactly my point. Time to get rid of the world's nuclear arsenal. By your own admission, we will not be 100 percent able to prevent a WMD attack, even if we have the missile defense system. And by your own admission, if even one WMD is delivered, even if it's not via a missile, and even if we have the missile defense system and we are not able to block all incoming missiles, the world will still be destroyed. So the only way to assure that the world will not be destroyed is to eliminate the nuclear arsenal.