100,000 Japanese civilians died in one night of bombing. And the govt presses on, and 40,000 civilians died on Okinawa. Japan still refused to surrender.
Not much better proof needed to show that Japanese leadership would've watched their population die of starvation over months and years.
Japan had no capacity to attack anywhere anymore. All of its fleet had been sunk. What was the point of leveling the country to the ground, when the opponent is already KO and on its knees ?
The point was to neuter the Japanese imperial system, which had declared war on and attacked the US, aligned with the axis, and spent the previous decade butchering Asia. That required total capitulation and the abolition of their State Shintoism, not a mere agreement to cease hostilities.
Given the vast genocide Japan had committed - given their fanaticism - there was also no way to know for sure that Japan would cease its war machine and territorial conquest aims, without invading and overseeing the dismantling of their system.
After what Japan had done, were the allies supposed to just take Japan's word for it, that they were going to become peaceful and withdraw from mainland Asia permanently?
A blockade, no fuel, no food, Russian invasion imminent unless surrender to the US. this is a big "What if" and I do not think we can say outcome A or B with any certainty.
Not much better proof needed to show that Japanese leadership would've watched their population die of starvation over months and years.