Google is constantly pushing their proprietary APIs so a lot apps in practice only work with play services, if you even can get hold of them without the Play store app. they also abandon more and more of the AOSP builtin Apps, new features and are added mostly only to the proprietary counterparts. So a lot of things people might attribute to Android are in fact part of the Google Apps. The direction is clear and AOSP is entirely controlled by google, they could simply decide to not open the next Version or parts of it.