The sweet spot for me would be something like Discourse or even an old-school PHPBB. Slack has a similar problem to IRC - it's ephemeral and the answers don't show up on Google.
NodeBB's a lighter and more performant alternative to Discourse.
It didn't, as long as your server didn't distribute binary newsgroups, which where the only thing requiring a substantial amount of space. Furthermore, servers had no obligation to keep a long retention history, and could limit it to a few months if they wanted so.
Yes, I vote for NNTP. In my previous life I used to run a local NNTP/Usenet/Fidonet gateway, storage was not an issue at all, but bandwidth was.
Some of the most interesting applications of NNTP would never connect to the legacy usenet network, which as you mention is/was a mess.
The NNTP distribution model was you could take whatever subset of feeds you wanted and keep them however long your storage and bandwidth allowed. Back in the day, a 56Kbps link worked pretty well. A T-1 meant you could pick up the funner binaries feeds. You don't need some multi-meg JS frameworks to read that content. Now, it works fine with GigE and multi-terabyte storage. There's even an RFC for running it over TLS.
Reddit is the modern replacement? Things are that bad?
I think Google Groups killed Usenet when they made a web interface for it. It was flooded with people who didn't understand how it worked. There was a definite turning point at that time, and I cancelled my subscription.
I do think it's a useful model for private discussions, with plenty of supporting software. It wouldn't be bad for internal company use. But it doesn't have animated emojis and trophies, so no one would use it now.
Google Groups used to be handy for shadowing a bunch of Usenet groups, but that's gone now. I'm not sure how much Usenet exists these days.
In all seriousness, Reddit is the modern successor. I dislike a lot about Reddit; their UI/UX is a contender for the worst in the entire world; and they provide a forum for a lot of the most unpleasant people in the world (not quite 4chan bad, but pretty bad). But at the end of the day, Reddit is where most threaded public conversations moved to.