Building a replacement from scratch is not within anyone's ambition. Google have done to it what they did to the browser, to maintain a mobile system from scratch would take a team of hundreds. Add to that, a system runs apps, as opposed to a browser, and so you either have to have a compatibility layer for android apps, or have no apps.

Someone could fork AOSP. The time to have done that would've been 2013, take android K and fork it, do not merge upstream. We could be living the dream right now. Someone could, theoretically, take that old code and go through the tedium of looking through every android update, taking what's needed for device compatibility, security patches, actual improvements like battery management, and porting those. That's a ton of work.

No ROM developer actually maintains an OS, they all just patch the newest AOSP version. And so we get stuck with Google's decisions about how we are supposed to use our property. You haven't been able to change the default DNS for years. I hear LineageOS has gone so far as to stop maintaining it's flagship features, like sliding the status bar to adjust brightness. It's no CyanogenMod.