So to you Quartz is just a Compositor.
In general, the term Quartz or Quartz technologies can refer to almost every part of the Mac OS X graphics model from the rendering layer down to the compositor. In this use, the term covers Core 2d, Core Image, PDF Kit, Core Animation and Core Video as well. https://developer.apple.com/referencelibrary/GraphicsImaging/idxQuartz-date.html
OTOH, X is "just a protocol".
Don;t know what's wrong with X? Read "Papers and Talks by Keith Packard":
I've read them. If you read them all, then you'll know that most of the later ones are about how they solved the problems identified in the earlier ones.
Yes but a lot more still need to be done.
Keith identified a lot of X's problems can't be solved at X's level and should be fixed at "above xlib" or whatever. Some of them freedesktop.org is working on. some were lost. e.g. color management system.
Keith also talk about compressed image transport.
Today, to display a jpeg file, an x client (eg firefox) decompress the jpeg and send the decompressed image to remoteX. If X is used oven ssh, the image will get jpeg decompress and compressed by ssh and decompressed again at remoteX...
While XIE is dead, I'm sure Google Chrome OS will get it right.