Japan didn’t surrender until several days after the atomic bombs. Militarist fanatics among them were willing to have everyone die in a glorious fight for the homeland. They were overruled by Hirohito and those who believed that Japan could surrender without losing face. The bombs gave them cover.
If Japan had not surrendered, estimated US casualties for the first 90 days alone of Operation Downfall were over a half million. The American casualty estimates for fully conquering the home islands ranged from 2 to 4 million.
No one wanted to do that alone if we could share the burden.
I agree with everything you said except the bit about sharing the burden. We shared the WIN when Russia had done nothing to earn it against Japan. Where were Russian troops anywhere from Pearl Harbor to Tokyo Bay?
Happily, Truman took the decision that he did. MacArthur favored the invasion which would have cost at least a half million of our boys.
Also happily, Hirohito wished to save what was left of his people.