Google is already well in the process of making everything they can proprietary. Browser, Calendar, Camera, Dialer, Gallery, Music Player, SMS and probably more - all left to basically bitrot in AOSP while Google pushes proprietary replacements as part of the gapps bundle loaded onto OEM phones.
The core of Android may be open source, but between so many API's being shoved into Google Play Services and AOSP apps being all but abandoned the writing is on the wall.
Exactly. They actually go one step further than that with the Google Services API's. Things like notifications and location can now go through the proprietary API's instead of the system ones, with special perks. This means that if you don't have Google Services, then even if you get your hands on a third-party APK without using the Play Store (hard enough as it is), things will sometimes just randomly break because API's are missing.
The core of Android may be open source, but between so many API's being shoved into Google Play Services and AOSP apps being all but abandoned the writing is on the wall.