Google Play is not just app store, but also the only way to efficiently deliver push notifications on Android devices. And latest modifications to Android OS make it very rippled without push notifications, all background apps unload from memory very fast. The result is that it makes many types of apps impossible to implement without relying on push notifications - and lock in to Google Play services.
While they carry almost the same name, Google Play Services are not the same as the Play Store, though. You can use the former without using the latter.
I highly doubt that a manufacturer can release an Android phone that does not have Google Play Store app but does use Google Play Services. We have evidence that Google prohibits even releasing other models w/out Google Play store if it wants to make Androids with Google Play.
Same is true for Apple, of course.