The reason you shouldn't assign 50% to the proposition "I will win the lottery" is because you have some understanding of the odds behind the lottery.

Yup. Similarly you don't assign 50% to the proposition "X will change", where X is a relatively long-lasting feature of the world around you - long-lasting enough to have been noticed as such in the first place and given rise to the hypothesis that it will change. (In the Le Pen prediction, the important word is "cease", not "Le Pen" or "election".)

ETA: what I'm getting at is that nobody gives a damn about the class of question "yes/no question which I have no idea about". The subthread about these questions is a red herring. When a question comes up about "world events", you have some idea of the odds for change vs status quo based on the general category of things that the question is about. For instance many GJP questions are of the form "Will Prime Minister of Country X resign or otherwise vacate that position within the next six months?". Even if you are not familiar with the politics of Country X, you have some grounds for thinking that the "No" side of the question is more likely than the "Yes" side - for having an overall status quo bias on this type of question.