Not a whole lot to expand on here. Posts that receive a lot of upvotes are never actually unpopular; the comment sections are almost always in agreement.

But people are supposed to upvote unpopular opinions and downvote popular ones

Very seldom is this rule followed. No amount of moderation or rule postings will stop people from downvoting a post because they disagree with it.

The time limit is arbitrary, but it’s based on the fact that most discussion is usually died down within 3 days. The amount of upvotes is also arbitrary. I’d think something like +50 would be a good maximum before a post should be removed.

do posts have to be removed? Couldn’t they just be locked to the same effect?

Maybe. Locked posts annoy me more than removed posts though. I’d rather just have them gone.