I think Discourse is the first real attempt to bring forums in-line with "modern" UI expectations, which is why it feels like it won. There's probably lots of room to grow here. There's forums out there that allow SMTP-only [1] or SMTP and NNTP reading/posting [2], there's forum skins atop mailing lists like [3], there's distributed forums like Aether or Lemmy like [4, 5]. Unfortunately these are all new/raw.
I really miss NNTP. I appreciate that spam was a huge problem, but it was really nice being able to discover and subscribe to a large number of topics and navigate them all from the same tool. And there was innovation in the client space.
Reddit is probably the closest alternative I know of today. But, several communities treat an associated sub-reddit as unofficial in favor of their Discourse instance. However, I simply can't navigate 20 different Discourse instances every day. Likewise, I can't keep hopping between different Discord or Slack workspaces/servers. Yes, they're in the same client, but I have to keep making expensive context changes to load channels from each server.
As a result, I've mostly given up. There are a few communities I'm attached to that I'll put up with the poor tooling, but the others are basically invisible if there isn't a sub-reddit. I'd suspect this has made communities more insular, even if the tooling is less obtuse than something like IRC.
With Discourse, you can enable mailing list mode and read all the Discourse instances from your email client. That’s what I do.
Personally, I only have one Discourse instance I keep up to date on, but if that instance weren’t right there in my email client, I’d have zero.
edit: The main caveat in my experience is that you probably want to click through onto the website if you’re planning to reply, both to be able to preview formatting, and to double-check that the post you’re replying to hasn’t been edited in a way that renders the reply unnecessary. (I wish there was a way to deliver edits over email.) But most of the time I’m just reading, and for that I just stick to my email client. The loss of edits doesn’t seem to be a big deal in practice.
Reddit certainly has its flaws. I suppose I've been fortunate that most of the sub-reddits I'm interested in have pretty good members. There's invariably always a jerk or two. I've run into that on Discourse, too. I suppose it's just something ingrained to online communities. I don't love Reddit, but it is the closest thing I've found to a central hub for multiple communities.
With NNTP, I could access the entire archive and it's separate from my email account. As far as I know, Discourse notifications only start from the date you subscribe, which makes it fine for new conversations, but isn't terribly useful when trying to avoid asking a question that's already been answered. Trying to be a good netizen, I search the archive first, but that brings me back to an isolated Discourse web instance.
Additionally, with NNTP, the "sign up" process is very low friction. With Discourse, I need to seek out each community separately. It's enough work where I'm unlikely to experiment with new communities.
I would argue that "modern UX expectations" is a large part of the problem with Discourse. Infinite scrolling is one prominent example. Wasteful whitespace is another.
Your mention of NNTP reading/posting caught my interest, but I wasn't able to find any mention of it on tade.link; is that perhaps a now-deprecated feature, or is it just not documented?
[1]: https://lobste.rs
[2]: https://tade.link
[3]: https://lists.wikimedia.org/hyperkitty/list/wikimedia-sf@lis... for example
[4]: https://getaether.net/
[5]: https://lemmy.ml/