You made an implicit assumption, and that assumption is very possibly wrong. You are “theorizing about the consequences” just as much as me by making that assumption.
For example, I can think of at least one way the US could’ve tried to avoid the huge civilian death toll: drop the bombs in the ocean, target the japanese navy, close enough that the blast will be seen from the mainland , yet far enough to avoid most civilian casualties. Then tell the Japanese to surrender, or else they’re next. I don’t claim to say it would’ve worked for sure, but at least they would’ve tried.