The 2016 election is indeed a one time event. But so is the toss of a die or the flip of a coin.
When someone claims they know a candidate has a 75% chance of winning they are not predicting the outcome. They are claiming they know the shape of the die.
The outcome of the election can't invalidate this. But if the model they use to arrive at 75% is tested against many elections it could be shown to have limited predictive value. Or it may be born out as valuable.
Of course, once a valuable predictor is known to the candidates they can change their behavior and the model can be made irrelevant. Or it be blown all out of proportion. Just look at what happens in Iowa.