GrapheneOS won't survive the next generation of devices because bootloader unlocking will also go away (>>44765939 ), and without kernel security updates that OS can't continue.
Now there's also no more sideloading, so what purpose does Android even serve anymore?
Can you download, build, and install a basic Android system these days without touching a single piece of closed code? Absolutely. Will it be able to do much without closed binaries? No.
Android isn't GNU/Linux where there's a general ethos of making everything in userland FOSS if at all possible. Rather, it's a free OS that both Google and manufacturers can do anything they want with, including shove a ton of spy and bloatware on it, then make it to where you can't get rid of those things, at least not easily.
The optimism from 15 years ago surrounding FOSS in the mobile space is on its deathbed.