Isn't graphene completely dependent on what Google ships though? Ok they may be able to support Motorola devices longer than Googles releases facilitate supporting Google devices, but it's still ultimately android, which is still ultimately controlled by Google.
I'm not saying it isn't an improvement but this feels like choosing centos because you don't like the direction fedora is taking.
Writing a new OS from scratch simply ain't gonna work, because people rely on Android apps working on their phones.