To your second point, I agree, but I was comparing specifically the common top posts ("front page") to one another. HN is obviously more niche, but the ways in which its niche is defined are what I contend do, in fact, result in a better quality of discussion.

To your first point, I also concede that this phenomenon certainly does happen, but would counter with the notion that this probably happens with every conversation ever, with it perhaps tapering off in materials science discussions amongst experts.

I'd assert that it's happening right now, given that neither of us are likely informed enough to empirically support our assertions here. We're merely sharing our rather vague impressions.