So if the Japanese hadn't surrendered after Nagasaki, the U.S. would have been justified in bombing every Japanese city to ashes in succession -- until there were no more Japanese people left alive?
What you've presented there is a case of post-factual justification using a rationale that only works in hindsight. Because the Japanese surrendered after two atomic bombs were dropped (as opposed to 50, 100, etc.), we can look back and feel vindicated that what we did was right.
“So if the Japanese hadn’t surrendered after Nagasaki, the U.S. would have been justified in bombing every Japanese city to ashes in succession — until there were no more Japanese people left alive?”
In a word, yes.
L