Why is it not better for Apple to make Swift truly cross-platform?
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 Swift compiler and standard library are pretty tightly linked. The Swift type system is getting pretty complicated already, handling a whole range of lifetimes, async, the combination of protocols, generics, existentials, etc. with some magic handling for arrays and maps in the library. So the cost of cross-platform effort would be significant.
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.)
Hope this keeps getting investment and support by Microsoft. Choice and competition are very welcome, even though i code neither iPhone Apps nor C# ATM.
Kotlin/Native deploys and works on iOS, but due to Apple holding back on opening up Swift, it has to do everything through ObjC interop. But make no mistake, to Apple, that's a feature: if other languages have a hard time integrating in their ecosystem, they'll go away.
Apple’s problem has never been the “barrier to entry”. They know developers will make apps for their platform no matter what. If Swift becomes more universal it makes it easier for Apple developers to also develop for Android (etc) which Apple would see as a bad thing.
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.