Calculating probabilities about nearly any real world event is extremely complex. Someone who accepts the logic of your post shouldn’t believe there is much value to Bayesian analysis other then allowing you to determine whether new information should cause you to increase or decrease your estimate of the probability of some event occurring.
It should be possible for someone to answer the following question: Is the probability of X occurring greater or less than Y? And if you answer enough of these questions you can basically determine the probability of X.
What if the alternative was for the U.S. to firebomb and blockade Japan for another year and then with Russian help invade. Over twenty times as many Japanese would have died, a million Japanese women could well have been raped by Russian troops, and one-have of Japan would probably have had to live in a communist dictatorship for at least 40 years. Would you really prefer this alternative to what actually happened?
Or even worse, what if the alternative was for the U.S. to use biological and chemical weapons on Japan and kill most of its population?