One thing to note, more and more of what constitutes the android user experience is being pulled into the Google Play Services app which is closed source. A big part of the reason why is that it gives Google a better negotiation tool to use with carriers as they have to license the use of the Google Play platform and that isn't really optional in modern Android right now. AOSP has been left behind not in support but in more and more features of "Android" being closed source. Another huge benefit is that tons of bug fixes that would have required coaxing carriers into supporting a software update on the phones can now be applied just by patching Google Play Services and rolling it out as an app update.