Syntax squiggles are not very important, but other language features, like a good design and removing whole classes of bugs, are.
If Andrew Kelly were kidnapped by aliens tomorrow, Zig would continue. I think that's the dividing point, when the bus factor is no longer one.
Elm, sadly, never achieved that. But I don't see why Borgo would necessarily follow the way of Elm and not of Zig or PHP.
Thankfully I've been able to scramble some syntax highlighting support and reverse engineer how to start and run it, and after a while it actually kind of makes sense.
But I am an engineer, not an artist, and elm code will not survive me in the code base if I can avoid it (unless there are teams of Elm programmers at a place).
Elm might remove classes of bugs, but when there is not a single developer except me who manages to wrestle with it, though luck. TypeScript also removes entire classes of bugs, and it does so while being well documented and easy to understand for devs from other mainstream languages so the other devs won't come running my way everytime they have to fix something written in it.
Technically not dead, rather stagnant, a difference that hardly matters regarding adoption and ecosystem future.