But being open source means that one could fork AOSP. Also the Google proprietary stuff comes with the Play Services, not AOSP.
So AOSP is a lot better than the alternatives, I would say.
I have a different feeling about Chromium: it is open source, but through it Google gets to control the future of the Web. But Android is not a standard, it's "just" one OS, so changes made by Google in AOSP don't really impact, say, iOS.