It's true, civilization could be smashed even now by foolish use of nukes. It almost happened before in 1983 and in 1962. More groups with nukes means more opportunities for disaster, though since the Cold War no nation has stockpiled ridiculous thousands of weapons.

Please cite a credible source for your idea that "anyone who wants a nuke bad enough" can get a former-Soviet warhead. Your gut feeling isn't very convincing.

Sort-of-on-topic for the thread: Ken MacLeod wrote a series of SF novels in the 1990s about about competing anarcho-capitalist and anarcho-socialist human factions colonizing space after many nation-states collapsed. One of the story strands was about an ex-Soviet republic that retained its nuclear arsenal and sold outsourced nuclear deterrence on the open market to groups who wanted to live under a nuclear umbrella without building their own weapons.