And if the Soviets had come in as “backup” for us in the ‘War for Japan’, they surely, too, would’ve demanded a big piece of the action (a la Germany being partioned). Imagine a Soviet Japan. Indeed, those bombs saved countless lives and that potential political tyranny.
Roosevelt was a communist sympathizer, and gave away several perks to Stalin. FDR coddling extended the life of the Evil Empire, and made them more powerful after WWII. Fortunately, Truman was not so inclined, and FDR was dead before the final showdown with Japan, or your observation might be history. FDR was developing the bomb in a race with Germany, not Japan. Germans were the original motivation and target, who knows what FDR would have ordered.