The argument doesn't just include American deaths, but Japanese deaths as well. You may not agree, but the us just finished in Europe far past the point where the war was effectively over, and there was the belief that the Japanese would always take it further than the germans. Part of it was the japanese war strategy. They wanted the enemy to think they would have to cut through them to the last man woman and child, to get them to give up. There was a belief that maybe a shock would do something, but there was also doubt to that. 7 more bombs were going to be ready to soften up japanese defenses during downfall. They thought they would have to do much more damage.

Edit: I messed up my words in a way that subtlety changed the meaning of a sentence.