An aspect that does is the position of a mouse on a desktop with the position of a cursor on the screen.

Except that the "desktop metaphor" stops being a "desktop" with the introduction of a cursor and mouse. Actual desktops are manipulated directly by users... and desktops don't allow you to move your hand all the way to the right and magically move to a new clean desk (in the case of virtual windows). There's nothing about a desktop that infers the behaviors described which is why it's crappy for conveying the actual behaviors of current UI... hence the limited nature of metaphor.

Given an appropriate physical implementation of a desk, more aspects the desktop metaphor could certainly be implemented. Typically metaphors allude to the platonic ideal of something and not any one concrete instance of a given subject.

I'm not sure that metaphors allude to Platonic ideals but rather (to stick with Plato) are closer to the shadows on the cave wall.