There are no problems for the open drivers except the work to do, but if the user want/need to have the best from his hardware, then the manufacturer's drivers must be avaiable for that display server.
In the case of the display server, the work to do is not on the hand of free/open developers only, so you cannot fill the gap only thanks to the volunteers' work.
So yeah, to me If we are talking about display server the things are different.
Different scenario happens if both the new display servers can be sodisfied through the same interface (i.e. EGL).
Well, the Debian mailing list now reminds me of scenes from Phoenix Wright... Objections aplenty. And someone wrote a summary of things that were less than stellar during the whole process: https://lists.debian.org/debian-ctte.../msg00476.html