Right, that's why I said "essentially". My memory is hazy, of course, but I remember that a whole lot of people stopped using usenet after that purchase. Some because they felt sold out to Google, and some because they saw it as a sign that Usenet was going to get absorbed into the borg. A system being decentralized does not make it immune from being that sort of thing. Look at what gmail did to email.
Whether or not that was most people, I don't know. But it was more than a few.
I mean, it was all around the same time. But I would argue it was a correlative effect, not causative. People were already moving on to web based message boards, more and more people were viewing Usenet as a place for warez & not discussion boards, etc. I'm sure there were people for whom that was the final straw, but I can't imagine that being the true nail in the coffin.
Personally I stuck with Usenet until the early 2010's. But it was so low signal that in retrospect I'm not sure why I bothered.
Whether or not that was most people, I don't know. But it was more than a few.