Will quantum computing forge the new Mutually Assured Destruction (MAD)?
I don't think so. Quantum computing should eventually pose a threat via Shor's algorithm, but then it will be one of those "have it or you don't" technologies. Eventually people will make the push to post-quantum cryptography and things will more-or-less look the same as they do today, I wager.
Cryptography safe from quantum computers has been in use for a long time. It is already replacing cryptography that can be broken by a quantum computer in theory and any feasibility of a useful quantum computer will only accelerate the transition.