Russia cannot be allowed to win.
But also, Putin cannot loose so hard that he actually reaches for the nukes (meaning either he needs to die or those weapons are first removed from use), and even without Putin there's a fear a collapsing Russia would disperse nukes on the black market and/or oligarchs would fruit into atomic warlords.
This does mean Ukraine destroying Russian nuclear delivery systems a while back was directly useful, makes it easier for everyone else to help them.
But even so, I have no idea how this plays out: Russia's death throes spraying nukes at the west is still entirely possible; as is Ukraine developing a nuke, pointing it as stuff Russian oligarchs like, and getting them to defenestrate Putin without Ukraine even launching the weapon.
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Other things to consider: qhich power grids, if any, can cope with a single nuke triggering a high-altitude EMP? Most extreme estimate I've heard says it would take only one to kill 90% of the USA in a year just from loss of electricity in too many places at once to repair fast enough.
How sure can we be that all post-Russian nukes get accounted for?