We've also made progress as a species towards banning and reducing other things that in-group upsides and really bad externalities: off-the-shelf sale of broad system antibiotics; chattel slavery; human organ trafficking; some damaging recreational drugs.

The prohibitions aren't perfect, of course (and not without their own negative externalities in some cases). But all of those things are much more accessible to people than nuclear weapons, and we've still had successes in banning/reducing them. So maybe there's hope yet.