individuals are meant to represent wealth or privilege, and intended to represent a means to an end as opposed to an end in its own light.  In this, Highsmith is making that argument that all individuals can, in a sense, be replaced because there is no treatment of people as an end.  The class and social configuration of the characters in the novel are defined by their status of wealth and privilege and in this, they are replaceable by other people.  This comment on class is a fairly cynical one, but it is one that enables the reader to understand how individuals immersed in this system view one another and view each other.  The only way for a Tom Ripley to not ascend through such a social configuration is if individuals are known for who they are and their character content, and not for their trappings of wealth and privilege.  Due to the fact that this world for Mr. Ripley is one where such elements define individual identity, he is able to slip into it and assume Dickie's place with relative ease, a statement of human replace-ability.