It's not its lack of features that people are leaving, it's that the rest of the Internet 'gained' features, specifically Facebook, Reddit, and Discord, and they consume more of every potential user's attention.
Sorry, no. I don't think you get it. People weren't leaving because reddit had threaded comments (or something dumb like that), and that was so great they just had to spend more time there. "Features" have nothing to do with the issues.
> and they consume more of every potential user's attention.
Metafilter's always been so small (and must remain so to work), that concerns like that are totally off base.
The last few years, maybe since 2016 really in some ways, have been really difficult on the site and people got used to more of a siege mentality in terms of managing megathreads, and very limited resources (human and otherwise) and that's a tough set of habits to turn around.
Please tell me more about why engagement has dropped precipitously. Here I thought it was the superior UI of other sites but I can't wait for you to tell me why I really left.