Your argument assumes that the time-horizon of rational utility maximisers never reaches further than their next decision. If I only get one shot to increase my expected utility by 1%, and I'm rational, yes, I'll take any odds better than 99:1 in favour on an all or nothing bet. That is a highly contrived scenario: it is almost always possible to stake less than your entire utility on an outcome, in which case you generally should in order to reduce risk-of-ruin and thus increase long-term expected utility.

Further, the risks of not using nuclear weapons in the Second World War were nothing like those you gave. Japan was in no danger of occupying the United States at the time the decision to initiate the Trinity test was made; as for Germany, it had already surrendered! The anticipated effects of not using the Bomb were rather that of losing perhaps several hundred thousand soldiers in an invasion of Japan, and of course the large economic cost of prolonging the war. As for the calculated risk of atmospheric ignition, the calculated risk at the time of testing was even lower than the 300000:1 you stated (although Fermi offered evens on the morning of the Trinity test).