I'm still really disappointed that the Cast protocol is effectively closed, and Google only supports casting from Chrome, Android, and iOS.

I think Google is really shooting themselves in the foot regarding adoption. I'd love to have a command-line Cast client for Linux, integration into pulseaudio, a Firefox Cast extension, a native UI (non-browser) video player, etc. Sure, some people have reverse-engineered the newer protocol, but I've never gotten any of the unofficial clients to work reliably (and some just flat-out don't work at all).