USA dropping nukes would have prevented the convention against using nukes in wars from being started. I think there's a pretty good chance we wouldn't have any civilization left by now if we went down that fork in history.
How is nuking Japan different from nuking Korea? Everybody agrees that forcing Japan to surrender with nukes was much better for everyone involved than a ground invasion.
When Japan was bombed, nobody else in the world had nuclear weapons, the US only had 2, and there were only a handful of people outside of the US seriously researching nuclear weapons and were still years away from a test. By 1950 the USSR had working nuclear bombs, had proven so with a nuclear test, and a dozen other countries had started their own nuclear weapons programs.