That idea works in theory, but in practice it makes comments a popularity contest. Eventually, people with minority opinions figure out that their ideas are unpopular and leave or stop commenting. The end result is monoculture. A great illustration of this phenomenon is Reddit's politics subreddit. In theory, it is a place to discuss American politics. In practice, its discussion is almost entirely centered around a single candidate from one party. The downvote button windowed the community and then winnowed it again until a political atom remained.