Isn't that the point and one of the general themes on the show. That the wasteland gets inside everyone and no one is immune. It shows that the BoS is not a "good" organization to viewers that might not be familiar with the lore. Despite their fancy ranks and appearance they can be as corrupt or self serving as anyone else in the wasteland.
Elder Quintus did promise that Maximus would be his "co-leader" if he helped deliver cold fusion artifact to them. So he wasn't promoted for doing nothing.