> That said, if given the choice between a tiny FOSS language and a tiny proprietary one (that might be profitable), I'll always choose the latter.

(Not to put words in parent poster's mouth, just for reader clarity)

F# is no more proprietary than Rust. F# was started by Microsoft Research but is now an open source language 'owned' by the F# Foundation and licensed under the MIT license (https://github.com/fsharp/fsharp).

(The historical Visual F# compiler was developed by Microsoft per http://research.microsoft.com/en-us/um/cambridge/projects/fs...) but AFAIK today Microsoft's small full-time F# devtools team work on contributions to the open-source compiler in addition to their work on VS tooling.