Earlier quoted context omitted.
From that link: Once Mir’s support for Wayland clients is on a par with the support for “native” Mir clients we will likely phase out support for the latter. Does that mean they want to phase out Mir once they have a full Wayland display server? I thought the whole point for them was to continue to support those that are already using Mir? This still doesn't make any sense to me. I thought one of the original distinc…
What existing wayland server? Gnome and KDE has it's own, there was not a generic server when Mir announced support for Wayland and from what I know some people are working on creating a library that can be used for creating compositors, Mir is a solution for DEs like Mate because it would be easy to work on using Mir as a server so gaining wayland support then doing a compositor from scratch or using a library.
I don't know what was around when Mir was started, but plenty exist today:
https://en.wikipedia.org/wiki/List_of_display_servers
It was always obvious to me that Wayland was the future replacement for X for a long time. Why they had to take the ideas in Wayland and do something a bit different in Mir has always been a mystery to me.