Earlier quoted context omitted.
But why is my window manager now also my display server? The (relatively simple) program that decides how to lay out my windows is clearly a separate concern to the (complex) program that actually interfaces with the GPU to render the screen Having to worry if your WM supports your GPU is like having to worry if your web browser supports your NIC
Gnome is not your window manager - it’s your display server. You can trivially install/write an extension to manage your windows differently and reuse everything gnome provides for example. A wm has always been trivial compared to a display manager, people just mistake one for the other.
Qt/KDE is lagging behind and there are no easy way for them to catch up. Had the display server been a single implementation, Qt/KDE can focus on WM/Widget..