Google can leverage a lot of control using Play Services. If, as gp suggests, they also create an OEM layer so the OEM doesn't have to fork Android and instead

Google can leverage a lot of control using Play Services. If, as gp suggests, they also create an OEM layer so the OEM doesn't have to fork Android and instead just builds on top of safe APIs, allowing Google to take over updates entirely, then Google can force OEMs to use it.