I took it that dianavan's point was that there were all kinds of consequences of a historical event like Hiroshima, and some people alive today wouldn't have been born if it hadn't happened and some people who weren't born would have been born, and would have been alive today.

For all artbrooks (for example) knows he or she wouldn't have been born if Hitler hadn't come to power, and the second world war hadn't taken place, since his or her parents would likely never have met each other. I don't think that'd be a good reason from thinking that on balance Hitler was a good thing.