emacs is surely on a decline, but it’s not obvious that Vim is on the same trend.

This matches “theory” and anecdotal evidence: the people who chose emacs probably didn’t like modal editing, and when “better” IDEs came along they just switched. But there’s nothing like Vim (except editors specifically inspired by Vim), and those haven’t gotten any traction if only because every one of us Vim users have hjkl muscle memory burned into our brains.