I think that’s an astute observation. And you’re right that it happens in GD as well as the Pit. It also happens in CS, as when two music theory nerds begin arguing over whether B# and C are the same note for all practical purposes, or when a couple of people monopolize the Big Brother thread. Hell, it probably happens in MPSIMS for all I know.
But it really doesn’t bother me when that happens. It’s like you say: they’ll duke it out until the end, or the thread will eventually just drop out of sight. The only problem occurs when someone happens to open the thread as a kind of Johnny-come-lately, and makes a comment. And then one of the hangers-on will scream, “Don’t hijack the thread. We’re talking about X.”. I’ve made the point before that just because PosterSmith and PosterJones are talking about X, doesn’t mean I can’t talk about Y, so long as Y is pertinent to the OP. So, no problem with the limited participation threads in the Pit. Just don’t exclude people who want to join in late.