I think graphene could maintain a hard fork with manufacturer support (Motorola). At least for a while. Long term viability is an open question.

They could for a while but what's the point if it's not sustainable? Google is not going to turn around and make it more open again.

With every Android release you will build up more feature base to replicate. Unless you cut all ties and drive a separate ecosystem but good luck getting enough developers to buy into that.