Why isn't Death of a Salesman a perfect tragedy?
The notion of a "perfect tragedy", or more precisely, a perfect exemplar of the genre of tragedy is based on a classical ideal first articulated in Aristotle's Poetics. In Aristotle's theoretical framework, the tragic hero was a person greater than average, possessing striking nobility of character, and a model for people to emulate. The downfall of the hero was due to a single character flaw that led the hero to err by doing one irrevocable act that led inevitably to his downfall.
Willy Loman of Death of a Salesman by Arthur Miller is not really a tragic hero. Rather than being a person greater than average from a noble family, who is a model for the audience with the exception of a sinle tragic flaw, instead Willy is a mediocre salesperson of the middle class, and a deeply flawed human being who partially redeems himself in one final heroic act.