Chromecast Audio makes good sense - it seems like Google's approach to Airplay and so allows a phone or computer to "cast" audio to a speaker. Apple TV and Airport Express do the same thing but, as Apple TV morphs into a more full-fledged console and given the Airport Express's wider mandate (router, printer hub, etc.), this is a reasonable slot to fill. And the 3rd generation Apple TV is still a little pricey (and big) to act merely as a speaker bridge.
A simpler "Apple Audio" dongle could respond directly but I still think an even more interesting approach would be to take the Airport Express (and Airport Extreme - which, bafflingly, does not have speaker output) and allow Apple Music to run directly on the router rather than simply act as a bridge between the computer and speaker. This would avoid lag and help battery life enormously.
