Nuclear proliferation, not US policy, has marginalized the MAD theory. At the heart of MAD is the expectation that combatants will 1) survive a first strike with the ability to counter attack and 2) behave rationally. Neither applies to N. Korea, Pakistan or Iraq (if it succeeds in going nuclear). MAD is no longer possible where the most probable threat resides.
I haven't studied nuclear scenarios in almost 20 years, but loosing 90% in a first strike seems unlikely. By the time relations with the USSR deteriorate to that level, we'd have our Tridents and air craft carriers at sea, B52s back up and circling, and perhaps be prepared to launch ICBMs on warning. Even in the exceptionally unlikely USSR surprise strike during peacetime scenario, I don't think there's any doubt that they would cease to exist as a governable nation the next day. You don't need to take my word for this. Just recognize that if this 2/3s reduction really did threaten our security, huge portions of our retired military would be protesting it, not just a few commander wannabes and fringe ideologies.