Re: "especially from an engagement and marketing perspective"
"the desktop has become a more iterative experience, a continuously developing thing which isn't necessarily bound by maintaining backwards compatibility."
Too true. Some of us like backwards compatibility in things like continuing to be usable, not chucking out functionality, not slurping data to a mother ship somewhere and not suddenly starting to show advertising. We also like backwards compatibility for the under-pinnings, the libraries which applications rely on to display on the desktop.