Is retrospective decision-making valid?

[ledwix]

Suppose a risk is associated with an event. The event has probability A > 0 of being extremely successful and making your life awesome. The event has probability B = (1 - A) of being a failure and making your life considerably worse. Now suppose that A is much, much greater than B. There is almost no chance that the event is going to turn out bad, and when you ask people for advice, you get resounding encouragement. From this position in time, absolutely everyone agrees that you should carry out the event due to its high chance of success. Now with this encouragement, you carry out the event, but you find that the section B of the sample space actually occurs, and so your life becomes much worse.

Retrospectively, you did something that made your life much worse. Does that mean you shouldn't have done it and that B was actually 100%? Or, given that the initial probabilities were actually correct, and that there is just no way to determine the result of a single iteration of an event unless 100% or 0% is the probability of the event, should you have actually done it still?

Also, I'm not asking if retrospective decision-making is useful. It can't possibly be useful if the initial probabilities were actually correct. Once you make a decision with good knowledge of the chances of its success, there is no way to go back and change things once you know the outcome. Instead, I'm asking if it's valid, i.e. should you have actually done it?