To me, those type of functions (dialer, messaging, music player, etc) are more "app" than "OS" - obviously alternate music, messaging, home screen, etc apps are

To me, those type of functions (dialer, messaging, music player, etc) are more "app" than "OS" - obviously alternate music, messaging, home screen, etc apps are available, but what about dialers? Is that piece of the system replaceable? I think it would make a lot of sense if the base Android OS (AOSP, as I understand it?) were only as functional as it needed to be for useful apps to run, and then the "Google app stack" or "Cyanogen app stack" or whoever else's, open source or closed source or anything in between, could be dropped in to provide the user-facing functionality. The key seems to be getting to a point where important pieces of that underlying functionality aren't wrapped up in the "Google app stack".