If Android were an open platform in the way that Firefox OS or Ubuntu for smartphones were an open platform, the forking suggestion would make more sense. The AOSP/GMS split wouldn't exist. Everything would be in AOSP, so piecemeal substitution of back-end services without having to reinvent vast tracts of code and without any major compatibility implications would be practical.
But it isn't. Not only is it not this kind of an open platform, but Google is actively working to make it functionally less open with each new release. The result is that a forker has to make a choice: they can give Google control and get the all the upsides of the platform, or they can snatch control from Google and get almost none of them.
Android isn't designed to be forked. With GMS, Google has deliberately designed Android to resist forking. Suggestions that Microsoft scrap its own operating system in favor of such a fork simply betray a lack of understanding of the way Google has built the Android platform. Peter Bright via daringfireball