The greatest threat that nukes pose to humanity isn't the initial blast. It might not even be the lasting radiation or the electromagnetic pulse that could fry our electrical grid. If enough nukes go off, we could darken the atmosphere enough with smoke to send the Earth into an anthropogenic ice age.
You might argue that the widespread possession of nukes deters the entire world from setting them off, but , there have been an astonishingly large number of close calls where misinterpreted signals or malfunctioning equipment caused either Russia or the United States to briefly think the other had already launched a nuclear attack. If a country winds up retaliating based on an erroneous signal, the nukes could start flying for real.
It would take thousands of nukes to plunge us into nuclear winter, but both Russia and the United States have thousands of nukes. If deterrence can be achieved with fewer, maybe further disarmament of nuclear weapons is the best way to safeguard against such a catastrophe.