I'm not sure Henry James believes that. If you want to be true to the spirit of the book, that is true to the spirit of the book. No matter how loyal the filmmakers' intentions, James cannot be transplanted intact from novel to film. Decisions have to be made. While there is room to digress in literature, in the theater stories must maintain their momentum.
Some characters do have to drop off the table.
I mean, I have 90 minutes or something. The reader has hours. The new film adaptation of The American contains several striking changes from the book, the most obvious being the film's opening sequence. I felt that placing the secret at the beginning would give us that trajectory. In the film, Newman hands the letter over to Madame de Bellegarde in exchange for her promise to permit his marriage with Claire.