I think they may be safer hires, anyway. They will be able to program themselves out of a wet cardboard box, which is more than you can say about the average Joe who may just be good at bluffing the interview.
The thing that would worry me about niche languages is the amount of wheel reinvention you'd have to do. Not sure the cost/benefit calculation starts to look so great any more when you realize that your enthused elm developers will have to build a whole lot of stuff that you can just import in other languages.
FFIs can have warts, but in the general case library support isn't the obstacle people make it out to be.
Interviewing costs money—good tech interviewing particularly so. The less of the people you have applying to start with, the more money you save on culling then by way of tech interviews.
Elm nicely abstracts over JS interoperability with a mechanism called ports: https://guide.elm-lang.org/interop/javascript.html
Someone who hello worlds new languages may not know the syntax or idioms to write fizzbuzz in any specific language, but they will be able to characterise the rough shape of the problem, and not just sit in silence...
"bluffer overflow vulnerability". :)
And don't get me started on the way people misuse Django.
I personally know of a story where a startup started with datomic and clojure, and it was a bad idea because datomic could not effectively delete things. Eventually they went the standard java and standard cassandra / postgres / redis type route.
[1]Languages like PureScript provide a much more convenient FFI for this glue, but the glue and escape hatch is still there with Elm.
F# and C# compile to the same intermediate language, are interpreted on the same runtime, and are produced by the same organization for the same IDE... Their libraries are compatible and they rely on shared access to the common .net ecosystem.
Your absolute worst case scenario would involve decompiling your F# source as C#. More realistically you would use dependency management to gracefully transition out of one of the languages.
Transitioning piecemeal works exceptionally well: it's exactly how I've transitioned numerous projects away from C# onto F# ;)
I'm kinda surprised more companies don't take the risk - its not like these are bad languages, either. There's still a huge chunk people (in terms of absolute number, not portion of total devs of course) out there playing around with languages like Elm or Haskell or a Lisp/Scheme, or OcaML, F#, etc. who'd be super excited to use those languages professionally.