This is not a problem with X. DPI and pitch information is accessible via the xrandr extension. It's GNOME and GTK that arbitrarily chooses to ignore that information.
The problem with X is that all X11 screens have to have same DPI (and few other things, like color depth). Which is quite a problem when you have mixed-DPI displays.
Now the separate X11 displays do not have this limitation, but they have other ones: like not being able to move your window from one to another, without restarting the app. Historically, there was only one app, that was capable of changing the display without restart: XEmacs. For all the others, I have my doubts, that users would accept such limitations.
Now, both Xinerama and Xrandr standardized on using X11 screens for multi-monitor displays for exactly this reason. With displays roughly with same DPI and the graphics becoming truecolor, it wasn't really problem. But nowadays, it is.
That is mixed-DPI hardware. Another issue with your argument is, that "DPI and pitch information is accessible via xrandr extension". Yes, it it, that is true. The problem is, that it is one-way street. The client can read that, then it can respect that and adjust its rendering... but it doesn't have to. The client is free to ignore all that. Now the display server has the problem, that it doesn't know, how the client decided: should it upscale the client, or not? Nobody knows, all the display server has is an opaque bitmap.
With wayland, the client must explicitly set the scale, communicate it do the display server, so then the compositor knows, how to handle that surface. It can even transparently downscale the surface, when the user moves it from HiDPI to normal-DPI display. Something impossible on X11.
