Usenet
as the principle locus of online group discussion phased out ... within a few years of the Eternal September. I'd argue that the shine was clearly off somewhere in the 1997--1999 timeline, which is roughly when the first discussion sites such as Slashdot emerged.
Transitions aren't instantaneous, and aren't always clearly visible at the time. (Look at, say, the present ... evolution ... of Birdsite, The Front Page of the Internet, and the StackExchange empire.) But in retrospect inflection points become more clear.
(This is pretty much a constant in all of history, with eras, transitions, and labeling of these coming years, occasionally centuries, after the events themselves.)
The Eternal September was one of a growing onslaught of insults to Usenet which it ultimately succumbed to. Not instantly, but in time, and inevitably.
