And what would they do ? Swim to California with knifes between their teeth ? Japan posed no thread to the US at that stage of the war. The conflict was over, the allies have won.
The allies had hardly won. Much of Asia was still under Japanese rule. The Japanese still controlled Singapore, Hong Kong, massive swaths of China and South East Asia, Korea, Taiwan, etc. The idea that the US could have simply "walked away" at that point is ludicrous.
I cannot remember the exact number but since the Japanese had so many islands as well as mainland Japan, the cost of invasion in lives (Western and Japanese) was massive.
Of course it was not just a numbers decision, there was certainly some posturing for the post war world.
The US was very concerned that Russia would not honor the agreements around splitting up Korea and roll on into Seoul.
Then you had Germany and Eastern Europe to think about.
It is interesting to theorize what would've happened after the war if the U.S. had not developed the bomb or if the U.S. had developed it but chose not to use it.
Nonsense. Russia would have occupied Tokyo within the month, maybe less, Japan was defeated, and thoroughly. The US needed Japan to surrender to them, not Russia, and so it was.
The USSR had no real amphibious capability to carry out such a plan.
Civilians were dying at a rate of something like a quarter million per month in all the various Japanese-occupied areas at this point in the war. If the atomic bombings sped up the surrender by just three weeks they were a net win in terms of lives. (And yeah, I know how morally difficult that calculation is, but that's war.)
The bombings were geopolitical stage props.