In what sense is Barthes justified in making the rhetorical claim of the death of the author?

In his essay "The Death of the Author," Roland Barthes finds justification for the idea of the death of the author by considering it to be an inevitable historical evolution for culture and literature. Specifically, he believes the French literary style of Modernity (for him meaning the early twentieth century) has forever changed the reader's idea of the meaning of language in a novel or other literary work. We as a culture believed in the Author before only because we believed a novel or other fiction had a single, fixed meaning every reader could get out of it. But, Barthes argues, the splintering and fragmentation of culture's belief in any kind of fixed identity (be it religious, political, or the like) following the First World War resulted in a new understanding of the language in a novel as having as many meanings as there are readers to interact with it. The Author can no longer exist when there is no one meaning or intent a reader can get from a novel.
To back up his claims with historical figures, Barthes cites Marcel Proust, twentieth-century French novelist, as being a major killer of the Author. Proust's narrator in his set of novels (which make for thousands of pages of extremely detailed autobiographical impressions) is only ever a man "who is going to write," according to Barthes. By this he means Proust's entire goal as a novelist was to write and write and write until he actually had something to say. Therefore, we can only look at novels now as creations in process, collected bodies of paragraphs, never allowing the idea of one clear interpretation of a book to be justified again.
Barthes believes that Surrealism too (and its frequent "abrupt disappointment of expectations of meaning") contributes to the argument that the Death of the Author is an historical inevitability, something that culture, through new movements in art, was finally able to accept.