I beg to defer on the device drivers. Maybe you don't remember, but it's been decades before Linux would adapt, reimplement or convince manufacturer to provide drivers for it.
Modems first. Graphics card next. Wi-Fi networks also. Winprinters. They all come to my mind. And it's only by time and effort that now looks just works everywhere.
And now while graphene indeed is doing a great job that I appreciate very much, at the same time they are not developing an operating system. Google is.
There is a huge effort behind developing a full operating system, and it also requires standardization and somebody who defines what the standards are.
At this point, Google is the only one doing that. And if they go closed doors, no open source AOSP clone could keep up with Google changing standards and still be compatible, which would end up as an incompatible operating system.
My point is that currently Android needs Google, and there is no fooling around. We are years away of being independent from Google, whatever the great effort other developers are doing.
I appreciate everybody's work and I have been a lineage supporter and maintainer myself.
There are tons of issues that we need to solve to be really independent from Google. Forking he's the least of those.