Disabling most is insufficient. All it takes is a few ICBMs hitting NY, LA, etc., and the calculation already becomes nonsensical. It would be intriguing, however, if it becomes possible in the future to intercept ICBMs during the launch phase using some kind of new technology. Even then, though, you would need to ensure a 100% success rate.
Major US cities being destroyed is pretty bad but not MAD bad. The rest of the US will still be around long enough to decimate Russia or China. MAD means both sides get wiped out completley not some horrible killing of 50mil people happened.
I.E.: if they fire first, firing back does not mean everyone dies.
