So: zero is my rounded estimate. The argument the destruction of civilisation would have ensued if Vasili Arkhipov had acted otherwise seems flimsy and insubstantial to me - and similarly for the other claimants. It all adds up to less than 0.5.
Are you asserting in the cases of Petrov and Arkhipov that events would not have turned into full scale war or that you think it is likely that additional events in the chain could have prevented that?
Self-evidently the destruction of the world requires the first and not the second. My estimate is based on the joint probability - and not on the consideration of just one factor or the other. So: since neither factor is insignificant, I reject the dichotomy.
how proximate a cause do you need before you think you can speak of someone saving the world?
It would usually be better to give probability estimates than to crudely divide the population into "saviours of the world" - and "everyone else".
Stanislav Petrov is a rather famous person (of course only on Lesswrong, not in the real world).
But there is another Russian who saved the world: Vasili Alexandrovich Arkhipov.
On this day in 1962, at the height of the Cuban Missile Crisis Vasili Arkhipov prevented the launch of a nuclear torpedo and thus a possible nuclear war.
It's strange that Petrov attracts much more attention than Arkhipov. E.g. googling "Stanislav Petrov" produces 101.000 results, "Vasili Arkhipov" only 9.040 results. By contrast searching for "Britney Spears" generates about 295.000.000 results. Sorta depressing.
Anyway, let this day be the Vasili Arkhipov Day.