Seems like they just put the comments with the most likes at the top... stands to reason that more likes = nicer comment.

First part of your comment is right, but your reasoning is wrong. A couple years ago, YouTube changed the visibility of negative comments, such that positive comments are prioritized (can’t like a comment if you don’t even see it).

This was also around the time they removed the thumbs up/down count.

This was to prevent creator burnout; imagine if every video you put out had some snarky diss against you in the comments.

I'm not sure. I would have assumed that this naive algorithm is what they had at first, and what gave us the cesspool?

I'm not convinced that the current trend of positivity is not caused by bots, but I wouldn't be surprised if the old system was just based on engagement. Meaning that upvotes and downvotes would both push the comment up. This is not intuitive, but would fit with what we've been seeing for the last decade or so.

Oh, some of the positivity is surely caused by bots.

And lots of the cesspool was not caused by bots.