GrapheneOS and SailfishOS focus on a narrow set of devices and they can keep up with hardware support. I agree that you have to make concessions in terms of allowing proprietary firmware blobs and opaque baseband hardware. You also have to choose your hardware wisely (e.g. GrapheneOS can/could piggyback on Google's driver work).

I was just saying that you can make the problem more narrow by not trying to support every device out there. Start small and pick your battles (which probably means using AOSP and using sandboxed AOSP).

I think the main issue of many previous attempts was what typically happens in the FLOSS community: there are N attempts rather than one coordinated attempt (Ubuntu Touch, Plasma Mobile, PostmarketOS, PureOS, etc.) and everybody is targeting different hardware. It's similar to how the Linux desktop got fragmented, though it's even more problematic for mobile, since the usage is probably 1/1000th of Linux desktop usage.

Yeah, the fragmentation is the main issue, however Firefox OS is a proof that even a single device doesn't work if there are no concessions, and the only thing left are unintesting hardware for the general public.