When things work further development is not always a positive.

That was also true back when it was actively developed though. X apps compiled a long time ago still run fine today.

Xorg is still being under development, there is another fork in development too (XLibre) and you're in the comment thread for a project about a brand new X server written from scratch.

Usually things which are "no longer developed" do not have (at least) three implementations in development.

Mature projects often don't change much or quickly even when actively maintained or improved.