This is basically what happened to Android as they moved an increasing number of essential features to Play Services.

To be fair, these are mostly integrations with other Google services. I wouldn't expect this to be an OS-level package.

Ironically, this is an example of where it is crucially important Oracle-Google case swings Google's way.

Currently, there are alternative implementations of Play Services that can be installed to replace Google's. However, if it is not fair use to use even the bare bones of an API definition without permission, then we can't even create a compatible implementation of such an API without the copyright holder's permission. In which case, we cannot replace Google Play services with anything else.

Actually, Google already has closed source features in Chrome that are totally not related to Google accounts and/or sync. Take for example Android app support in ChromeOS. It is closed source even though both Chromium and Android are open-source.