I may have a different Theory of Internet Commenting, because to me not-replying is the default and I need some particular reason to continue a back-and-forth. Do I have something worth contributing to the conversation - something worth adding or clarifying? Is the discussion heading in a worthwhile direction, such that I want to take it further in that direction? Or, is this one of the (relatively rare) cases where I have a social obligation to respond?

There are many exceptions, but in a typical discussion I think that:
1. There are rapidly diminishing returns to extending the conversation (I usually communicate most of what I have to say in one or two comments)
2. The discussion is primarily for the benefit of the many silent people reading the comments, not the few particular people who are making the comments (so I don't feel much need to press my interlocutor on what they think, or to clarify exactly what I think, unless it adds something to the discussion as a whole. One cue I use: if a comment has been upvoted then it probably represents many people's views, and is more worth replying to)