One piece of history that Ms. Campbell oversimplifies: Wonder Woman was not the JSA's secretary during the entire early run. She first appears in All-Star #11 and disposes of an entire Japanese invasion force, and has other independent adventures before she's relegated to secretarial duties. No pro from that time period ever commented on the changeover, so anything we say today is speculation. However, there is a rumor that Wonder Woman's creator, who had some authorial control of his character, may have ticked off All-Star's editors by changing an assigned script-- after which he was "punished" by having his character relegated to secretarial status. I guess it's sexual discrimination in a roundabout way, but it doesn't quite fit the paradigm presented.
As I said, it's speculation-- but I see nothing else that explains why Wonder Woman dropped her secretary duties in JSA after Marston passed in 1947.