http://developer.android.com/about/dashboards/index.html and no, fragmentation's not an issue for anyone with the Play Store and the compatibility kit (and maybe sherlock).
I mean, are you not aware of Firefox OS which bootstrap(ed/s) itself on top of Android? Or that Ubuntu for Phones is coming out with phones to support it?
Or of the Nexus program? Or the pseudo-Nexus program with devices with unlocked bootloaders.
OH and webOS still exists, and is open source and runs on some Android devices.
What more do you want, sounds like your needs are being met. Take the android kernel and write a new UI. Then you can run it on one of the what, hundred different devices that CM supports?
>They could add C, C++, Golang, Java, C#, even ObjC to the mix this way.
lol, you think writing and supporting all of the Android APIs for all of those languages is feasible? Or you want some kind of bindings or what exactly?
>Carriers can add apps etc but there's no reason they have to adjust the base OS, and both should negotiate with Google for inclusion of new drivers
and you don't seem to actually know who controls the distribution of the drivers