I'm not old enough to have experienced Usenet. For those who remember Usenet: Do you believe that Eternal September was a failure of cultural transmission? Or was it more of a human capital thing? (Early adopters/academics tend to be better-behaved users?)

Another way of thinking about it: Suppose the exact same userbase had joined Usenet, but they trickled in gradually instead of flooding in all at once. How much would that have changed the outcome?

FWIW I used to think it was human capital, but have updated more towards cultural explanations. For example, it seems fairly common for academics to make fools of themselves online nowadays.

I think it was the rate of change. Previous waves were small by comparison and had been absorbed/integrated after a period, with a degree of gradual change. Nothing is forever, but Eternal September was abrupt. To a degree there was an echo of it (not on USENET, but on forums and other places) with the advent of the smartphone in 2007, and another with the mass take-up of Internet services in 2016 on the Indian subcontinent by a huge population.