New comments have to compete with old ones on the page. Once a page hits one million words it is pretty much done, unless there's a mechanism by which it gets edited, condensed, summarized, or updated.
One technique that has been tried is giving every visitor the power to edit, summarize, or update the entire page. That's the Wikipedia model. But, of course, you lose the history of the conversation [1], and the individual voices, and you're subject to the editorial whims of whoever happens by.
Another idea is to keep the content in the form of discrete comments but allow visitors to rearrange the comments. That's kind of how Stack Overflow works. The HN voting system also serves to rearrange comments on the page. These things are kind of indirect, though, and they do nothing to deal with the volume problem. Words take time to read; you have to cut down the supply somehow, and that inevitably requires some rewriting as well as cherry-picking.
Or you could just periodically archive the discussion and start it over with a fresh page, ending up with a series of discussions, each lasting only a day or two, but possibly related to or built on predecessor discussions that stretch back into history. This is pretty much how HN works.
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[1] Yeah, there are edit logs. Most people don't read those for fun. Most of the events in an edit log are tedious and unenlightening, like reading raw Postscript source.