That the best thing for humanity would have been if we failed to avert disaster during the Cuban missile crisis.
Play the scenario out. US and USSR both launch. But stockpiles were not as large as official records because both sides lied to scare the other. 20% of the remaining failed to launch. Somewhere around 50 from both sides and everyone decides enough is enough. Most of the northern hemisphere is uninhabitable to humans. 80% of humans on earth die in the initial exchanges. Nuclear winter kills crops. Things get lean for the survivors but they make it. Climate change is stopped dead.
As we learned from Chernobyl Wildlife actually finds radiation more tolerable than humans. So 50% of the world becomes a wildlife preserve.
The knowledge of civilization continues but everyone has a visceral knowledge of the danger of hubris. Weapons of mass destruction are totally outlawed. Invention of new weapons is viewed as a type of mental illness and treated gently and firmly with a straitjacket and a padded cell.
Humanity has a firm slap in the face but science and technology continue to advance. Human civilization enter a slow and cautious rebuilding era and we achieve a lasting, stable, calm civilization.
Now snap back to the present. Wild hubris, refusal to acknowledge the massive existential risks we are blundering into at high speed. If humanity survives the next hundred years I’ll be shocked. If civilization ends in the next 30 years I won’t be surprised in the least.
Nobody says it because it’s unthinkable, but objectively speaking, for our species, for every other species on earth it would have been better if we’d blown ourselves off our high horse decades ago.