My father was getting ready to attending landing craft school before the A-bomb was dropped on Japan. He was in the Coast Guard and was going to be one of the guys who operated the landing crafts.
According to one estimate I saw, a conventional invasion of Japan would have cost 5 - 6 million Japanese lives and one million American casualties.
Many people are still buying wartime propaganda: an invasion of Japan would cause millions of casualties. Japan was defeated at the time of the bombing. There was no need to invade. A blockade would result in them starving. Of course there were hardcore holdouts. However, they eventually would have had to give in without an invasion. The Japanese were exceedingly cruel. Who was not? They should apologize to the Chinese when the Chinese admit to killing 60 to 100 million of their own people. They did not surrender unconditionally. They were allowed to keep the emperor. Had they been allowed to keep Korea and Manchuria tens of thousands of Americans would be alive today.