Why would it decrease over time? I could see the amount of unknowns increasing over time just as much as it could go down.

I agree with the idea that you can't not do something because of unknowns. That doesn't mean you have to do it though. You simple take caution because there might be a lot of unknowns. e.g. in the Agile development world if there are a lot of "unknowns" you just increase the story point therefore signaling that the story/task is more difficult and could take longer.