AOSP is used in many contexts like embedded devices where somehow enshackling it would screw up Google's self-interest in other market areas (like ensuring there is a wider population of Android developers).
But regardless, thirdparty ROMs will continue to exist regardless of how much effort it takes because the demand exists and will not merely dissipate.
Demand exists for a lot of things that don't exist. There's no technical reason that the internet couldn't have stayed mostly decentralized, with everyone hosting a node at their house, controlling their own data and feeds, with an experience not much different from current services and social media, and people would absolutely want this if the experience was reliable and had good UX. But so much effort has been shunted to centralized services that it never materialized. The demand is there, I've asked many people. Just one counterexample.
I'm working on a project myself but it's taking forever considering the large scope. Getting close to having it ready for technical individuals to try it out.
