If EGL can be used on the desktop, is its window manager interface functionality intended to abstract the differences in GLX, WGL, and whatever the Mac OS equivalent is ? Or could it replace all 3 of those together?
Yes, that is the intent. EGL started with the GLX API and then was genericized to replace X Windows-specific concepts with (hopefully) portable equivalents. EGL has also been evolving into a more significant role as the resource manager when using multiple Khronos APIs together, especially through the EGLImage family of extensions. OpenGL / ES, OpenVG, and OpenWF all can interact with or make use of EGL already, and there is work underway to extend that to more of our APIs.