GrapheneOS is at Google's mercy. They were alarmed when google changed the way they released the source code for the pixel 10 recently, and while they ultimately were able to deal with the change, it's causing them a lot if extra work.

If google ever wanted to, they could completely screw over Graphene, since Graphene is ultimately reliant on Google playing ball.

A pure Linux phone would not be reliant on a big corporation to exist.