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