These Elm subscriptions sound like "half" of Rx, and from what I can is still 'missing' thus far some of the "higher order" Observable tools such as filter, chain, map, throttle, et al.
I also offer an analysis of "higher order" observables in https://youtu.be/DfLvDFxcAIA, and the big difference is the underlying thread architecture you get using these two.
I hope I'll be able to make these sorts of things clearer in time!
I'd love to make the Elm jump, but it still has the Haskell stigma (not enough engineers making hiring difficult, too 'academic', etc) where my clients (enterprise) won't let me dev with it. They do however let F# in some projects. MS should do what they did for OCaml -> F#, but for Elm -> something_I_can_say_is_first_party_MS_supported.
This really hasn't been the case for RxJS at all. It's been praised and version 5 is largely a community effort rather than just an MS effort. You got Ben Lesh and Andre Staltz putting in the most work and neither are MS.
The only thing from MS people don't seem to give a chance is .NET. Everything recent (VS Code, open sourcing stuff, Chakra engine, etc) has been met positively and not dismissed.
I almost had a Microsoft-first shop let me use Haskell when I pointed out how many of Haskell's lead developers worked at Microsoft Research UK.
As for the Microsoft "first party" F# equivalent for the web, that would be Typescript. Typescript's typing isn't yet to the rigor of OCaml, much less Haskell or Elm, but it's getting better every release.
Functional langauges are great, dont get me wrong, if i had the choice id probably give 'elixir' a 'rust' ;) but i make do with my existing c# knowledge, sprinkle in some Rx, isolate my side-effects in closures when pure functions cant be had, and im pretty happy with both the look, readability and maintance burden my code generates. The advantage of C# here over a first order functional lanuage is my ability to customise the syntax to make things self-documenting as well.