The UI changes aren't "selling out" in and of themselves, but I don't care for them. I don't really mind how it looks aesthetically, but I like some of the functionality that was removed (page title at the top of the window, back/forward/reload/home all in the same location, status bar statically located at the bottom of the screen, menu at the top left, etc). The other major changes (integration of the social API, Hello, Pocket, and Reader Mode) aren't directly related, but it feels like a flurry of somewhat-controversial changes on Mozilla's part, so I think that a lot of people probably associate them with each other.
Australis relies on add-ons to re-add functionality that was removed. I think that's actually in keeping with the previous philosophy of the browser (lightweight core, extensible with add-ons/extensions). The other things should probably be some of those extensions, rather than integrated into the base browser.