I'm not sure mandatory comments would solve the problem. We'd get plenty of the "I disagree." comments. Could those then be downvoted? Does downvoting those upvote the parent? Could I downvote both. Etc.

I don't know the current threshold but increasing it doesn't create an echo chamber. Look at any thread about gun control in the US and you will a lot downvoted comments on both sides and presumably a lot of highly upvoted comments on both sides as well. Not only that but you can only downvote comments until they reach a certain age. You can upvote them much longer (perhaps forever?). This creates a bias toward increasing votes which itself is a good reason to occasionally adjust the thresholds.