One possibility for no A-bombs was the original possibility (very small, so was ignored) that they thought the bomb might ignite the atmosphere. Now hoe about if someone made a mistake in calculation and brought the possibility higher - say to 33%. Now with a 1/3 chance of killing everyone on the planet if it were used, I dont think they'd have tried it.
This way, all the other discoveries stay the same, we just lose the bomb.
This way, all the other discoveries stay the same, we just lose the bomb.