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Re: inflation for example, I think a lot of people assumed that since it hadn't happened in a very long time, it was less likely to happen soon
Who thought this? I guess people thought inflation was unlikely to happen super soon before the pandemic, but that was because they weren't expecting such a massive, unpredictable event to come and demolish the economy for a while. And I don't think that expectation was unreasonable. Having a low credence in something like a global pandemic was rational, and just because it happened to be wrong in that particular case doesn't prove that it was bad thinking.
If there was a good reason for an informed person to expect the stock price tomorrow was going to be higher, the market (aka aggregate of all people with opinions about the stock price) would drive it higher today.
But elections are nothing like this. People aren't trying to predict how well the GOP will do in the future when they choose who to vote for today. Elections are still somewhat unpredictable, but I don't see any analogy between them and the stock market. And I definitely that past performance is a predictor of future election results, in the short term at least. A significant change in the electoral results either requires a significant portion of people to change their mind, a significant change in the composition of the electorate, or a significant change in what the parties stand for, and none of those things happen overnight.