When your top reason for a language being mainstream is "interoperability with .NET", I think the argument is a stretch.
I don't think Scala, Kotlin, or Clojure would have had as much adoption if they hadn't had access to the JVM ecosystem of libraries available.
While it's not the only benefit, I think one could just point at the usage of OCaml as the alternative to F#. While both are in the lower percentages of language popularity/usage, I've worked with at least 50 (dozens lol) people who were paid to write production F#.
There are some hefty businesses built on top of OCaml so it definitely can be done, but it sounds too expensive to get a small business up and running if the code itself isn't the product.
So that basically leaves Scala, Clojure and F#
Erlang, Elixir, and Gleam are hosted on their own platform, and lots of use cases can be covered that way.
Also, Flix is a thing, and some Scheme/CommonLisp/StandardML implementations compile to C.
Ecosystem there. Purescript, Gleam and others compile to JS.
Oh, and WebAssembly becomes valuable as compilation target as well.
Also: Kotlin is almost a functional first language like F#