The challenge I have with this, and pretty much all 'alternate history' foreign policy discussions, is that they try to argue an indefinite (Japan would have surrendered if we hadn't done this) against a definite (We bombed them and they surrendered immediately). There isn't any way to prove that stuff of course, and you can tweak the indefinite to be whatever you want to overcome the definite. So in some ways it is…
Great observation, summarizes one of my main quarrels with people trying to reimagine history.