I mean, that's debatable. But in any case Xlib, Xaw, and Motif can be completely bypassed nowadays--XCB and Wayland are complete replacements for Xlib (with EGL

I mean, that's debatable. But in any case Xlib, Xaw, and Motif can be completely bypassed nowadays--XCB and Wayland are complete replacements for Xlib (with EGL finally providing the needed XCB-compatible alternative to GLX), and GTK+/Qt are alternatives to Xaw and Motif--while Win32 is still the only way to do many things on Windows. Granted, the vast majority of devs use wrappers around Win32, so they don't have to care about the low-level plumbing.