Every single thing you've mentioned can be fixed with Xorg/X11.
(except subpixel antialiasing because that already existed for a long time, i'm not sure what you refer to)
Then why hasn't it been fixed?
In open source, those who make the code get to make the decisions--for better or for worse.
My personal opinion as to why X11 gets so few contributors--the build system is so terrible that nobody wants to touch it anymore. I suspect they would get a lot more contributors if they changed to something like meson/ninja which doesn't need to recompile the universe to be correct.
Are you referring to something else?
And I still see autogen.sh and Makefile.am goop in the top level of that repository.
However, I would be quite happy to be wrong.
I don't know, my guess is that people found making new stuff much more fun than fixing existing stuff.
Which is fine.
However that isn't the same as the existing stuff being unable to be fixed.