It's the whole 'nukes' issue that the crux of it all. It won't end the world (the only thing guaranteed to do that is when the sun expands and swallow this planet whole) but our existence is conditional.

Even then there's the psychological aspect of if you survive the initial exchange, a lot of people can suffer from 'survivors guilt' and the complete destruction of infrastructure, so we REALLY don't want to witness that but idiots have run the pentagon for decades.