Mamet is by no means the first to create a story in which salesmen seem to symbolize the disintegration of the American Dream, but he does manage to pull it off without beating the audience over the head:
In Death of a Salesman, Arthur Miller made the salesman into a symbol for the failure of the American dream. In Miller's play, Willy Loman was out there all alone, on a smile and a shoeshine; Glengarry Glen Ross is a version for modern times. (Source)
These are men who were told that if they went out and worked hard, they could succeed. Instead, though, they find themselves always at the whims of those who are truly running the show:
MOSS: We have to go to them to get them. Huh. Ninety percent our sale, we're paying to the office for the leads.
These are guys who, for all intents and purposes, are busting their butts every day to try to make it, and they still have to fork over the majority of their earnings to the bigwigs in charge. On top of that, they have, for the most part, become broken down and bitter or utterly ruthless. This is Mamet's version of the American Dream… at least in this play.