This seems inevitable in the evolving post-Cold-War geopolitical environment. MAD only works when there is a nation-state whose cities you could conceivably target. When the threat model is a larger number of non-state terrorist organizations that do not have well-defined borders, permanent populations, or even stationary bases, having a 450kt warhead targeting a city is basically useless. Imagine that say a Chechnyan separatist group detonated a nuclear warhead on U.S. soil. What would the response be? Nuke Moscow? "Yes, please, we never liked them anyway."

The unfortunate thing is that everyone saying that this dangerously lowers the threshold for use of nuclear weapons is right. Deterrence works when there's an oligopoly of nuclear powers. When nuclear weapons proliferate beyond a handful of powerful nation-states, into countries or groups with nothing to lose, there's nothing stopping them from using their nukes. It's like how in the tech patent world pre-2010, people described all these "defensive" patent portfolios as mutually assured destruction, "and then Nokia launched the missiles".

IMHO nuclear winter is a far more likely end of civilization than global warming is.