The thing is the stuff that used to be total black magic on sendmail (address rewriting, setting routes up etc) is all pretty straightforward on any of those other mail systems, so I'm struggling to see why you would keep a sendmail install around. If someone called me in to help them with their sendmail, pretty much step 0 would be to uninstall sendmail, install a reasonable MTA and go from there.

I get the feeling that are maybe 5 huge sendmail users still out there somewhere that still pay for support contracts and that is just about enough to pay for someone to turn out regular updates. But beyond that they have reconciled themselves with the fact that they are never getting any new users.

I virtually lived on Usenet between roughly 1996-2006 and hung around a bit for a couple more years after that. I can assure you that Usenet definitely went somewhere. Even by 2005 Usenet felt pretty deserted by everybody other than the warez scene.

I will concede however that I haven't logged onto an NNTP server for probably a decade, so perhaps Usenet has had a resurgence without anyone telling me.