Assuming you were casting locally held audio files from a device on your home wifi network to the CC Audio I wouldn't imagine it'd require any access to the internet at all and would only use the local wifi network. However, presumably the CC downloads updates automatically in the background.
I feel old. There's too much choice.
While I do love the infinite choice and discovery of things like Spotify, I also don't like the constant shifting and redundancy of stuff.
As long as I have electricity, when I put a CD or MD in my hi-fi it plays. I don't need an app and it works even if my WiFi drops.
The other thing I hadn't considered is whether a Chromecast uses data as my folks have a limit.
At the moment I'm thinking just a regular neat midi / mini system with enough spare ports to plug in a Chromecast / future equivalent and a deck.