A "conversation" on HN is something with a diagonal shape (a comment replying to a comment replying to a comment, etc.) Adding a comment to something (a post or a comment) that already has other comments doesn't "disrupt" the conversation, it branches it. This isn't a phpBB.
I think you're mistaken. In this discussion, I actually found it quite disruptive to have to scroll past 60 comments talking about politics so that I could find the discussion about the thing we're actually here to discuss.
They have lots of places on the internet to talk about politics. If you want to do so, I bet you'll have no problem finding an appropriate place.
There's only the one thread, with those 60 comments in it—so that's really just an artifact of the fact that comment threads can't be collapsed on HN like they can on Reddit. With that addition, this system really does support as many tangents as the userbase cares to follow for any given post, without any egregious annoyance per user, up to a certain scale.
Past that scale (probably 1000 individual threads or so; rarely seen on Reddit, and never on HN), a secondary voting mechanism purely to sort threads by "on-topic-ness" might be required, or perhas an automatic sort on how similar in word usage the comments are to their parent, to derive some crude sense of topicality.
