I reluctantly stopped using mercurial 10 or so years ago, and reading this brings a little tear to my eye. Mercurial was just about where jj seems to be now. It had revsets. It had just gotten the concept of mutable and immutable commits. It had absorb. It had bookmarks. It never had an "index". The commands had human readable options like I see in this post. It could work with git repositories. Man I miss it.
I'm hesitant to pick jj up in case it ends up losing to git like mercurial did. But it's very tempting.
According to the guy who wrote JJ, he copied all the ideas you mentioned from hg. That included a lot of ideas from hg's add ons. So the similarities are no accident. But then he added a twist - he didn't just delete the index, he dropped "hg commit" as well.
I can't see it going anywhere. It is in many ways "just" a different porcelain for git. The plumbing is the same. It's also safer to use: no JJ command can lose data another JJ command can't recover.
It can't really lose to git, because underlying it is git