Ah yes, the US Strategic Bombing Survey, how could they ever be unbiased in concluding that bombing people to smithereens inevitably leads to surrender. No, I'm disinclined to trust their opinion.
We in the modern day have the benefit of being able to pull examples from more than just WW2. Before WW2, the predictions of air power theorists were that strategic bombing would end wars before the powers could properly start mobilizing (look up Giulio Douhet, this was seriously argued). Japan in WW2 is the closest any example of strategic bombing has ever come to compelling a surrender, out of a dozen or so attempts. Its exceptionality is itself a suggestion that maybe bombing's role in Japanese surrender is perhaps inflated.
People also underestimate how much death came from our firebombing which would have continued, and would have come from blockade-induced starvation, relative to the nuclear bomb deaths.
It's not like the numbers would have been lower...
This is a fair criticism (although the generally accepted story is precisely that 'bombing people to smithereens' with atomic weapons led to surrender), but the evidence they based that opinion on is generally available. Having looked at it myself, I wouldn't have said everything they did with quite the same level of confidence, but I agree with their broad strokes opinion that it was most likely possible to achieve a Japanese surrender in a reasonable time-frame without dropping the atomic bombs or invading.
My sense of it is that there was a strong feeling of the need to punish the Japanese, quite apart from any military necessity. I think that, combined with the need to demonstrate the power of the new weapons lead to a reluctance to seriously engage with the Japanese diplomatically. I take as evidence of that the fact that a key sticking point in the surrender decision was the status of the Emperor, who ultimately wasn't removed by the US anyway. The allies could have made their terms much clearer much earlier, and they could have engaged with the Japanese attempts to seek peace much more. And of course there was the ongoing worry that the Soviet Union might gain too much if things weren't sorted out quickly.
From Truman's diaries at the time:
> Discussed Manhattan (it is a success). Decided to tell Stalin about it. Stalin had told P.M. of telegram from Jap emperor asking for peace. Stalin also read his answer to me. It was satisfactory. Believe Japs will fold up before Russia comes in. I am sure they will when Manhattan appears over their homeland. I shall inform Stalin about it at an opportune time.
We in the modern day have the benefit of being able to pull examples from more than just WW2. Before WW2, the predictions of air power theorists were that strategic bombing would end wars before the powers could properly start mobilizing (look up Giulio Douhet, this was seriously argued). Japan in WW2 is the closest any example of strategic bombing has ever come to compelling a surrender, out of a dozen or so attempts. Its exceptionality is itself a suggestion that maybe bombing's role in Japanese surrender is perhaps inflated.