Swift doesn’t suffer this issue — it’s been built for multiplatform use from day 1. There are still some discrepancies, but today Swift on Linux is about as usable as Swift on Apple platforms. There’s even several (4+ last I checked) modern web frameworks for Swift and even GTK+ bindings.
On top of this, its Windows port recently reached a useable state and there’s work being done on a Win32 bindings. Once that’s done, it will be possible to write an app with a common core and 3 different UI layers all in Swift which connects to a web API written in Swift. That’s already leagues beyond what was ever achieved with Obj-C and momentum is continuing to build.
You could use Java/OpenJDK or even Kotlin/OpenJDK.
You could use dotnet core.
You could use Node.js with Typescript.
All those are mature and support all major operating systems way better than Swift does.
I'm not sure what Swift's Unique Selling Proposition is. From where I'm looking it seems to be: "it's backed by Apple".
Eh, I wouldn’t say that. I don’t see it significantly ahead of say GNUstep is for Objective-C; Swift on any platform other than macOS is still pretty much a second-class experience.