And this is part of why I don't like Marion. The other part is that he would insist on top billing in any movie featuring actual heroes. Men of real accomplishment are secure enough in their self-image to exercise restraint and express humility. "Wayne" could appear in a war film featuring men who had bravely fought in the very war depicted and he, a career actor, would always get lead credit. He could have, occasionally, taken a step back and allowed better men to have the limelight. It's a credit to Jimmy Steward's character that he would continue to work with The Duke and just that matter slide. Steward could have absolutely humiliated Marion if he had wanted that guy out of the way.
Besides that, "Wayne" was a lousy actor. "The Conqueror" is example of his prowess.