If Google kills AOSP, a lot more than just GrapheneOS will stop being able to exist lest some entity maintains a fork that diverges from Google's path. Vendors that aren't shipping in line with Play Services and the rest of the ecosystem as well as LineageOS and other custom ROM development teams will suffer as well.

This kind of decision would essentially kill adoption of Android in a good number of countries within a few years as well as be the end of Android adoption for anyone that cares about security and/or privacy. Yes, It would either kill or put a large burden upon GrapheneOS as a project, but that is also true for so many other projects in the ecosystem. If the developers shutter their AOSP usage due to upstream abandonment, the users will likely follow in the same pattern.