Google could conceivably stop contributing to the AOSP and make future changes private, but the FOSS licence won't prevent others like Graphene and Chinese OEMs from continuing development on their own. I believe Graphene made the deliberate decision to only support Pixel devices because these used to be "reference devices" and have unlocked bootloaders, but I saw somewhere that they are in talks with an OEM to make their own devices.
> I believe Graphene made the deliberate decision to only support Pixel devices because these used to be "reference devices" and have unlocked bootloaders,
From my understanding the biggest reason is actually security features that only the Pixels can meet. I'm sure the reference device thing is a factor though so I don't think you're wrong