Admittedly, if you're already using private torrent trackers, you'll probably find more missing releases by working towards joining higher tier trackers than usenet alone has. Usenet is more worthwhile for people without high enough bandwidth to build the ratio needed to join such trackers.
Zedstrian
While 'the paywall for access' argument can be made for usenet indexers (which just index NZB files, not the releases themselves, with most still allowing up to 5 downloads a day via free membership tiers), in contrast to torrenting, usenet providers cache newsgroup binaries (the actual releases) on the servers of their respective usenet backbone. Because of that, releases that might run out of active seeders on public or even private torrent trackers after a few years are sometimes available for significantly longer on usenet.
Edit: While it's not an excuse for the usenet indexers, rather the providers, usenet newsgroup binaries are downloaded directly from the servers of providers, and are thus not P2P like bittorrent is.
For some shows I've noticed that it's not too difficult to get around half of their seasons in 4K, with the 4K releases for the remaining seasons being seemingly nowhere, whether it be on public trackers, private trackers, or usenet. Doesn't seem to be an issue of shows only being shot in 4K after their first few seasons, as in some cases the 1st and 3rd seasons may have 4K releases without the 2nd season having one, for instance.
While long term availability seems to be a problem for lots of reality TV shows, usenet or a private torrent tracker like TorrentLeech should work if the show you're looking for is popular enough. As datacap-free usenet access is usually at least $50 a year ($52 a year for Frugal Usenet + NZBGeek; forgoing the latter would reduce it to $40 a year, but might take a while for several seasons of TV shows since free indexers only offer up to 5 downloads a day each), private torrent trackers would ideally be better, particularly given that they often bundle episodes into season packs. As public trackers have a lower proportion of seeders in comparison, they generally only have the latest seasons of most reality TV shows.
Unfortunate that they purge their database of unseeded torrents. While there wouldn't be the same incentive as on a private tracker for people to come back and reseed something, removing them decreases the likelihood that niche releases stay available in the long term.
Don't have the *arr stack setup myself, but as to managing a library of of releases with both English and French dubbed audio I've found that looking for releases with 'Multi' in their names to be a relatively effective solution, though in trying to find releases on a handful of public French trackers looking for releases with 'VFF' (didn't see standalone 'VFQ' releases, though some VFF releases have VFQ audio tracks included as well) worked in that case as well. This seems to only work for popular movies, however, with TV shows and less popular movies, while sometimes being dubbed into French, seemingly not having those audio tracks anywhere I've looked (suggestions for good French torrent trackers would be welcome!), with the likelihood of finding English dubbed audio for releases originally in French appearing to be even more remote.
If you find a release with both audio tracks but want to use them with the video track of a better optimized monolingual release of the same movie, if the video tracks of two releases are the same length, or you can determine the exact offset between them, you can use MKVToolNix GUI to re-encode your preferred combination of video, audio, and subtitle tracks. A bit tedious compared to the automation of the *arr stack or simply keeping the default multilingual release of a movie, but at least it allows for fine-tuning individual releases on an as needed basis.
If you decide to make use of usenet at some point, I've found that some usenet indexers, such as NZBGeek, show in their search results the languages of additional audio tracks included in multilingual releases.
How often does abtorrents open registrations?
Unfortunately as most usenet indexers aren't usenet providers or vise versa, as far as I know there isn't a way for indexers to know which backbones have cached a given release or not. If any are able to deliver that it would be Easynews, which is both an indexer and provider, but even then that would just be for releases indexed by Easynews itself.
Without a high enough upload bandwidth (only have 5Mbps myself), even with freeleech not counting against one's download for the ratio it's unlikely that peers will stay connected to download seeded releases when other people with far higher upload bandwidth or seedboxes can maximize their download bandwidth. Not a problem for everyone of course, but the 100-torrent method works whether someone has an upload bandwidth of 1 Mbps or 1000Mbps.
A usenet download can either fail due to being taken down after a DMCA or similar request, not being in one of the newsgroups cached by one's usenet provider, or because one's usenet provider has fewer days of retention than the age of the usenet release. In terms of telling a binary newsreader program (i.e. downloader) such as SABnzbd or NZBGet what your provider is, there's a page in the settings of those and similar applications to enter the domain name, port, username, and password associated with any usenet provider subscriptions and/or block plans you have.
The monetary barrier to usenet is probably why torrent trackers would be more popular, and thus more likely to be blocked by national regulators. As to why you're far more likely to get an ISP letter for torrenting than for using usenet, while the bittorrent protocol means than if you download a release you're also seeding it to others, paying usenet providers for access to their cached releases means that you're only downloading releases from them, and not uploading anything. Usenet providers do sometimes have to remove releases from their databases upon request, which is why paying for providers on at least two nodes can help in mitigating the odds of a release not being available.
Comparing the cost of a VPN to Real-Debrid + a Usenet Indexer + A Usenet Provider depends on which services you choose, but in the case of ProtonVPN , NZBGeek, and Frugal Usenet it comes to $72 vs $84 a year, with the latter being more if you want to add a backup usenet block plan from a different node (block plans have a one-time upfront cost and last until you use up the plan's download capacity). If you forgo the additional block plan and NZBGeek, instead using a combination of the free tiers from indexers such as Tabula Rasa, NZB Finder, Miatrix, and DrunkenSlug (most allowing for 5 free downloads a day), Real-Debrid + Frugal Usenet is the same as ProtonVPN at $72 a year. Also note that Real-Debrid is able to cache torrents on request as long as someone's currently seeding them on public torrent trackers, and that with usenet to download a release that is X days old, you need a usenet provider with at least X days of retention.
Frugal Usenet is on the same backbone as Usenet.Farm, so a better block account option would be Newsdemon on the UsenetExpress backbone. Apparently the Frugal Usenet year plan comes with a BlockNews block (Omicron backbone) as well, which seems to have a retention length closer to that of Eweka.
Edit: My mistake, only the Frugal Usenet bonus server is on the Usenet.Farm backbone.