Cooler heads? Kennedy blockaded Cuba, a direct military threat to the Soviet Union.
I think interdependence is a bigger reason it wouldn't happen. The major nuclear powers in the 1960s were largely self-sustaining, and wiping part of the map wouldn't have had much an impact. At worst we may have had some dependencies on third world countries for raw materials in some of the same sectors we had in WW II, like rubber
Now? Even a six month major disruption in economic activity would bring even the US to its knees as we can't make much of what we need at home, and its probably worse elsewhere. The US has the know-how (probably) to jump-start its manufacturing base given a 3-5 year strategic commitment to investment, but we would need to operate at WW II levels of rationing and economic intervention.
There's also the question of elite status -- the elites are in a powerful position in terms of economic status and political power, there's no telling what even a limited nuclear exchange would do to them. A handful may become more powerful, but it seems more likely that a large number would lose their status forever, either due to the economic disruption or due to outright nationalization of assets and the promotion of national security/military interests.