Why does xorg still exist?

I am familiar with the development history of xorg. I am not learned as to the history of failed projects to replace it. What replacements were proposed before Mir and Wayland and why did they fail? They couldn't have been that bad. Wayland is as bad as xorg if not worse by its lack of standard implementation alone and it received financial backing.

Desktop OSes will be dead before the “year of the Linux desktop.”

This is right and wrong. The OS of the future seems increasingly platform agnostic and so solely desktop OS platforms probably will not increase but many entrants from mobile will in a sense become desktop platforms in the next ten years or so.