For the time being, AOSP and Chromium are still open source, so why not piggy-back off of all that labor and development to provide what GrapheneOS users want at minimal cost and effort?
I'd say the biggest pain point is that Google Maps doesn't work because of the lack of Google Play Services. The missing Maps/Play Services also breaks apps that rely on the maps API. Most of them just fall back to not showing a map at all while the rest of the app functions normally, but it's still an inconvenience. For turn by turn navigation I switched to HERE Maps which works without problems.
sailfish itself was great though. admittedly the android compatibility layer really helped though
If Google cuts development of AOSP in favor of some closed-source alternative, the GrapheneOS team could simply continue development of AOSP on their own.
There exists a possible world where a group of underpaid FOSS devs forked Chromium and AOSP and effectively developed it further.
But it is not our world.
> the GrapheneOS team could simply continue development of AOSP on their own.
They won't be able to do so.
No need to "build a complete from-scratch alternative OS" when that was already done 30 years ago.