This is untrue when you're talking about app development, which this is about. It can be demonstrated by simply pointing out that Microsoft's, Amazon's and Baidu's Android derivatives are fully functioning commercially even though the Google Play Services are not available on these devices. The Amazon App Store even has 240,000+ apps.
There's an increasing amount of propietary Google software running on Android phones, but that has no consequences for other apps. No functionality of AOSP is being absorbed into Play Services at all.