Well if you look at the "cross-platform" support compared to the other languages such as Go and Rust, Swift is quite poor at this: Apart from Linux, platforms such as Windows, FreeBSD, WASM, Haiku, etc are all unofficial ports which community members have to maintain but it is getting there, slowly.

In terms of the prospect of a cross-platform GUI, SwiftUI is taking a similar direction to Flutter and React Native. I see no other serious alternatives from the likes of Go or Rust at the moment. As far as Apple is concerned, they have total control of the direction of the language and they pick and choose which components to open-source.