I'd look to the server side; JavaScript is still quite popular there even though you can use any language.
I'd expect the reverse to happen as well to some degree since currently backend devs are somewhat "forced" to take on JS if they want to do any frontend work. Or does that seem less likely to you?
I think the “backend dev migrates” case is less likely, in a purely math sense. The number of backend devs forced to do front end is smaller than the number of frontend devs.