They depend on the Play Service and Play Store, that's for sure. But I'm pretty sure they know it's a risk already.
Don't get me wrong: they are locked-in, that's a fact. And to be fair they benefit from all the work of Google on the OS. But that's not a reason to desire to go further and lose even more control.
They already do; Google's flavour of Android adds plenty of proprietary components on top of AOSP.