One thing I am a bit upset is how Apple approaches SwiftUI / Combine, as these two to me, are foundational pieces to the Swift language ecosystem. Particularly:

1. There is no way to bundle SwiftUI / Combine to the app so you can ship to earlier iOS versions, making development exclusively done on newer OS version. More over, for any bug fixes on SwiftUI / Combine, you have to wait for the OS updates for the fix. Just updating your Xcode is not enough.

2. While the SwiftUI required language features are all upstreamed to Swift language, and core developers have been patient to articulate why. If SwiftUI / Combine development was done in public (i.e. open-source), bug-fixes and language improvements will be much smoother.

To Apple, point 2 probably is a big leap. But they've done Swift open-source release, and it works relatively well with a sizeable open-source ecosystem on other platforms now.