I think using the bombs with the information the US had at the time was justified. From the information the US had, Japan had a quite credible claim that they would "never" surrender unconditionally, though lots of Japanese people and military units would surrender even if Japan itself officially did not, making it not literally "never". With nuclear bombs the US could have kept hitting them with increasing levels of force, probably not possible with conventional bombs. If the war had continued until 1950 most of inhabited Japan would have been destroyed, and the US would have won even without a single surrender.