If it happens or it doesn’t then it must be 50/50 odds
Some time passed, the event did not happen. Same person said: ok, 50%, either happens or not.
Some time passed, the event did not happen. Same person said: all my previous predictions were correct, now let's apply Bayesian update: (1-0.9) * 0.5 / 0.5 = 0.1, so now posterior belief that the event is likely to happen drops from 50% to 10%.