How so? I can indeed target every layer of the software stack using Swift, today.
E.g. ClearSurgery[0] is written fully in Swift, including the real-time components running on the Linux boxes.
I don't know why anyone would want to use Apple tools if they are not developing for Apple platforms. Apple barely maintains compatibility for their own platforms, using Swift on a non-Apple platform is setting yourself up for doubule pain.
That it's designed for a thing and becoming the go-to choice for that thing can be far apart indeed.
It just works. One language. Many platforms. Incredible performance.
With a simple tooling. No ugly script. Everything is naturally integrated.
I _can_ do the same with Rust, doesn't mean it's "the language I reach for" for making e.g. a website. Because the tooling, ergonomics, hireability factor, etc. are still very harshly against it.
Same with Swift, but I'd call that more of a wasted opportunity because Apple, unlike Rust Foundation, has a mountain of money to make it happen, and yet they don't seem to care.