After the nuclear fallout these would be the only areas where sustained human life would be possible.
Even a recovering climate wouldn't change much about that.
People grossly, grossly overestimate the long-term effects of fallout. The main negative effects of nuclear warfare would be infrastructure destruction and the collapse of food/medicine distribution - fallout wouldn't even register.
At the height of the Cold War, a full-on strategic exchange between the US and the USSR wouldn't have ended human civilization. Things would've been unpleasant in parts of North America and Eurasia for a time, but all these extreme scenarios that are sensationalized wouldn'tve come to pass.
Look at Chernobyl - it's essentially a thriving wildlife reservation, now.
Hiroshima is not the reference for later generation nuclear bombs, which were magnitudes above them by payload and blast energy and different in isotopes emissioned. Bikini Atoll is still uninhabited as it deemed unsafe even though the population was originally re-relocated.
Problems might also arise from incomplete reactions which might happen with cheaper, less sophisticated bombs or intentionally with dirty bomb types meant for scorched earth tactics or to maximize economic impact due to radiation related sicknesses.
https://en.wikipedia.org/wiki/Bikini_Atoll#Return_to_Bikini_...
All of which is of very little relevance to discussing modern nukes intended to cause fallout.
Mutually Assured Destruction refers to the combatant countries, after all, not to the human species.
I'd expect any MAD plan to include airbursts for big cities and ground bursts for breadbaskets.