I do not think it is fizzling out, but it has never been a reliable medium for all shows for a variety of reasons. As said elsewhere, you probably need more than one indexer, and probably also need a torrent indexer or two too (with download client as well obviously) to occasionally fill in the gaps.
For newly broadcast North American shows, there is almost never an issue with Usenet, but for older ones (or even current series but back a few episodes), things do disappear because of DCMA takedowns.
For British shows though, Usenet is not a reliable medium, but then neither are Torrents as there is just not the community of uploaders there are in North America, and those there are, seem to prefer other platforms to share (like Google Drive) that are not easily indexable and auto downloaded… but that is a while different thing.