There are downsides. If people start adopting a cross-platform subset of Swift, and Apple adds features to Swift which objectively require special platform sup

There are downsides. If people start adopting a cross-platform subset of Swift, and Apple adds features to Swift which objectively require special platform support (and other platforms don't have it), people will not adopt these innovations in their apps even when targeting Apple platforms, out of fear their previously cross-platform Swift code will no longer be cross-platform. This effect ties Apple's hands behind their back in their ability to guide and steer the platform's evolution going forward. They'll be adding improvements and no one will be using them. Of course, since Swift is so tightly integrated with Objective-C and Cocoa already, Swift is already quite Apple-specific. And that's what Apple really needs. A good language that makes sense for their platforms. We already have various incarnations of Mono and Java and what not compiling to iOS if people want cross-platform code. But the lesson is there are always two sides of the coin here. Remember when Android being "open" had no downsides, either? Witness the malware, fragmentation, and Google having to write really nasty contracts with their OEMs in order to be able to steer the platform in the right direction for Android's own good.