The standard library is becoming cross-platform as we discuss it here.
I'd argue that it's too late already, the ship has sailed, Swift is heading the same way as C#.
Is it just that the maintenance cost is not worth it? Or would it threaten its ecosystem in a way that I don't understand?
It would seem that making Swift cross-platform would make Apple's ecosystem more accessible to developers. The barrier to entry would be lower if developers only needed to learn the APIs and not a whole new language and its tooling too. This would help the ecosystem stay healthy for a longer time.
The greatest barrier to entry is XCode. It's archaic, impossible to extend, and has a very wide surface that would overwhelm any development team.
(Java works fine on macOS, but developers want to deploy to iOS.)
[1]: https://learn.microsoft.com/en-us/dotnet/maui/ios/cli
[2]: https://docs.avaloniaui.net/tutorials/developing-for-mobile/...