You have to consider that the purpose of a forum is to have a discussion, not to post articles, dissertations and theses at each other. That most comments carry low relative intellectual gravity is a feature, not a bug. Comments don't exist to provide "value", they exist to facilitate communication between posters.
And it's a fundamental aspect of the threaded nature of the forum that comments deviate from the central topic and become more narrowly focused and relevant to fewer posters as a branch diverges. I supposed if they switched to a purely linear format, where every comment was explicitly to the OP, then that might focus threads more, but they would also be less interesting.
But why 'less interesting' when comments are meant to be explicit to the OP?
Wouldn't the fact that there are ample opportunities to post diverse topics through multiple individual posts, provides even more ample opportunities to comment in-context on each one of them individually?
Contrast this to an offline world scenario, where let's say, you are in a setting with limited time on your hand. Your conversation (on any given topics) will start and end at some point because you are not going to be around together for long. In such offline contexts, you naturally may have various continuous topics to discuss and it is fine if things diverge.
But online, you technically do not have such time restrictions. Plus, you have a platform which facilitates contextual discussions which can stay neat and tidy. My argument here is, that we tend to not take advantage of the real value of the online forums (by being all over, as if we are not going to get the second opportunity).
I tend to think that the concise comments online, on an exact topic at hand, provide collective value, as opposed to diverged sentences or two. This does not mean that 100 people would write counter-article and/or dissertation on the OP (that would really be boring for sure). But instead of 100 short and diverged comments, 5 focused comments (counter, etc) are better overall in providing better value for the time.
And it's a fundamental aspect of the threaded nature of the forum that comments deviate from the central topic and become more narrowly focused and relevant to fewer posters as a branch diverges. I supposed if they switched to a purely linear format, where every comment was explicitly to the OP, then that might focus threads more, but they would also be less interesting.