You wonder why Google does this, and not just put content on the Roku. They have for Youtube, but the obvious reason Google is going this direction is numbers. Roku, at most, has...
Strange to mention Google Docs, which does work offline.
Weak sauce. It's true that Google's ad servers "track" you, but so do Microsofts. They will also allow all sorts of third party companies to track you too, via the real time bidding platforms....
You should have mentioned how notifications are actionable too.
Spotify has a web based interface now as well, so you can use it on Linux and your favorite OS, ChromeOS ;)