I keep forgetting about binaries.
Even if your ISP dropped NNTP, there were still free NNTP servers around, so I'm less than agreed about that expectation. And the Eternal September was in 1993. Usenet was "viable" well past ten years past that.
I think the best explanation is "new shiny Web" although about... half of the folks I used to read on Usenet are now on Other Fora and it's nowhere near the same - it's not a substitute.