I really wished they would choose Swift. It's open source now, and many developers would definitely enjoy having to learn only one programming language for developing native apps.
Unfortunately, corporate policies and hubris matter more in these things.
And the massive technical undertaking of making the Android framework work with a none Java interoperable language?
Kotlin was already mostly working with Android thanks to a community effort. This announcement just gives it Google's blessing and gives you all the Kotlin tools by default in Android Studio. Going forward they'll likely be working closely with the Kotlin developers, but this was primarily community driven.
While developers would love that, kotlin works because its interoperable with java. Swift doesn't have that. Try to add it and you'd shatter the swift ecosystem. Don't add it and people wouldn't adopt it because there's no path for gradual transition.
https://blog.jetbrains.com/kotlin/2017/05/kotlin-on-android-...