What is the theme in Death of a Salesman by Arthur Miller?

The ending is pretty much depressing. I think Miller was really drying to deconstruct the theme of the American dream through Willy. As readers, we quickly understand that Willy's version of the American dream is largely an illusion. Success, simply for its own sake, becomes a self-fulfilling failure. Willy wants his success quickly and easily without passion or pride. The spoils of his "success" never materialize. As Willie's illusion disintegrates, so does his identity built on that illusion.