"Swift is intended as a replacement for C-based languages (C, C++, and Objective-C)."
"Swift is a successor to both the C and Objective-C languages."
https://developer.apple.com/swift/
So while Swift might not be yet there, if Apple tomorrow releases a system that according to those statements, the only available SDK language is Swift, anyone that wants to be on that system will use Swift, regardless how serious challenger it might be.
As another example, are you aware that Brillo was supposed to be a pure C++ based variant of Android, before they pivoted it into Android Things? Now you can even write user space device drivers in Java.
Microsoft has also adding been adding low level features of System C# into C# roadmap, and UWP model (which they really want to be the future) only uses .NET Native. How far they will push it, who knows.
Windows 10 has a new UI composition engine, naturally it is programmed in DirectX with C++, however all the APIs are exposed as UWP projections and so far, every single demo I have seen used C#.
The big difference between OS companies and the FOSS world regarding systems programming, is that regardless how we think a given language is fitted for a given purpose, they can make it into the only door to the system.
But you are right they are clearly not yet there, and it remains to be seen how far they would actually bully devs into using only those languages.
It's those "few use cases" you are talking about that interest me.
Cedar had a GC-free, type-safe subset that used the typesystem for memory safety.