I think, George, that you have stated a corrolary of an old proverb. On usenet it is called "imminent death of the net predicted", and it's so popular it has its own dictionary entry:
Someone in 'The New Hacker's Dictionary wrote:Imminent Death Of The Net Predicted!: prov.
[Usenet] Since Usenet first got off the ground in 1980--81, it has grown exponentially, approximately doubling in size every year. On the other hand, most people feel the signal-to-noise ratio of Usenet has dropped steadily. These trends led, as far back as mid-1983, to predictions of the imminent collapse (or death) of the net. Ten years and numerous doublings later, enough of these gloomy prognostications have been confounded that the phrase "Imminent Death Of The Net Predicted!" has become a running joke, hauled out any time someone grumbles about the S/N ratio or the huge and steadily increasing volume, or the possible loss of a key node or link, or the potential for lawsuits when ignoramuses post copyrighted material, etc., etc., etc.
Source
I think something similar is going on in the political threads. On the one hand, I see more and more posters coming in. On the other, it seems almost all the quality posts continue to come from the usual suspects, while the new ones mostly contribute noise. As time goes on, people get familiar with the usual suspects, and can usually predict what the other usual suspects will say, which is boring. But if I browse back to the old political threads, I don't find them that much more engaging as the current ones.