Re:Building an Operating System for the Raspberry Pi
by pjmlp on Friday January 19, 2018 @12:53PM
(#16188184)
It is definitely the intent of Apple, explicitly mentioned on their developer documentation as C and Objective-C replacement.
"Swift is a successor to both the C and Objective-C languages."
https://developer.apple.com/swift/
As mentioned in another comment, most of the time the issue is political, not technical.
However, to the extent that swift (and obj-c!) are used for systems programming, those programs are useable outside of apple platforms.