I'd argue that nuclear warheads have saved millions of lives by preventing another world war. MAD prevents nuclear weapons states from waging war against each other directly.
You realise of course that in the alternative universe in which this was not the case, you'd be unable to make this statement.
That is generalisable to all global catastrophic risk: by virtue of the fact that they can occur once, and once only, means that we're effectively living in the survivorship-bias universe in which they did not occur.
That thought's only recently occurred to me, though Richard Posner wrote of it.
By definition, all but the last doomsday prediction is false. Yet it does not follow, as many seem to think, that all doomsday predictions must be false; what follow is only that all such predictions but one are false.
-Richard A. Posner, Catastrophe: Risk and Response, p. 13.
