It is true that drama is art imitating life and, for this reason, the credibility of a character will undoubtedly help a piece of living art succeed and transcend its time and circumstances.
The review of Death of a Salesman by Brook Atkinson in Death of a Salesman (1949) in On Stage: Selected Theater Reviews from The New York Times, 1920-1970 states that the play is successful because
It is so simple in style and so inevitable in theme that it scarcely seems like a thing that has been written and acted. For Mr. Miller has looked with compassion into the hearts of some ordinary Americans and quietly transferred their hope and anguish to the theatre. Under Elia Kazan's masterly direction, Lee J. Cobb gives a heroic performance, and every member of the cast plays like a person inspired.
This is very true when we think about the nature of the weaknesses and strengths that make up each character of the play, and how their environment play a strong catalyst for their changes.
Willy Loman, a man in his sixties and who should be thinking about spending his latter years in comfort and with his family, is a man who cannot achieve his full financial, professional nor emotional potential because he fails at the most important thing in life: To listen to his true voice and going with his true passion. In turn, he sells his life away to a get-rich quick type career as a salesman that merely ends up showing the true emptiness of a life without a true purpose. Willy is every American man who ever dreams of a better life in a quick amount of time. He is every man who fails at listening to his true voice. An audience can certainly relate to Willy and even understand his nature. This is why his character succeeds and transcends: It is three-dimensional and universal.
The characters of Biff and Happy, a product of their father's mentality, are also quite believable because reality shows us how generations affect each other and drag along their angels as well as their demons. In the case of Biff, we see a man who does experience the epiphany of finding out the truth of his life, and who suffers greatly because it is he who now will have to break, for once and for all, with the "curse" that is the fantasy world of Willy Loman: A world who has dragged down an entire generation. This aspect of Biff touches the psyche of the audience, and inspires the inner part of all of us who wonder if we, too, will ever uncover the true meaning of our existence.
Finally, the character of Linda, the submissive wife who lives in denial of her family's downfall, may seem old-fashioned to the modern audience, but she is representative of the woman who gives it all for her family and gets nothing in return. This is also a realistic aspect of humanity, especially when we live in a world where the woman still carries the burden of the success or failure of the family unit. Linda is also every woman: She is every woman who ever loved unconditionally.
Therefore, the characters in Death of a Salesman are mirrors of the American culture, and inspire us to look within ourselves and find the Willy, Linda, and Biff inside each and everyone of us.