Thanks for a well worded thoughtful post. Indeed there many 64-bit distributions which already run well on the Pi. In my opinion, most people literate enough to know the difference are also likely able to load something other than Raspbian on their 64-bit capable Pi. For those that can't, there is enough help on this forum to get a 64-bit distribution running. For example, when I had a problem with the 64-bit Gentoo image (WireGuard wouldn't build because of missing kernel headers) help came within a day.
In my opinion, the likely reason there is so much effort spent maintaining an official distribution like Raspbian is because the goals are different than mainstream distributions such as Ubuntu, Red Hat, Suse and Debian. In particular
- Raspbian includes software such as Minecraft, Mathematica, Scratch and Sonic Pi useful for education.
- Raspbian is designed to allow card-swap compatibility between Zero computers used in embedded projects and more powerful models used for development.
My point, however, is that Raspbian is unique enough to warrant the resources needed to maintain it. On the other hand, mainstream 64-bit ARM distributions are already well supported by their own development communities and don't need much help to run well on the Pi.