"As far as I can tell F# is one of those things where every single user is extremely happy" Isn't it because language has rather small community of passionate people, who are devoted to their language of choice?

F# popularity is somewhere between CHILL, Clipper and Raku langs, that are probably as obscure as F# for typical software dev.

I know Raku from Perl fame, and F# because it’s Microsoft, but CHILL and Clipper are totally new to me, so in my own humble experience these two latter look far more obscure. :D

I'm pretty sure that there's more production code written in F# than in all those other three combined.

If we consider number of jobs, it's probably on par with Rust.