[@BCTheEntity] Marines ranking up happens by seniority, as I understand it. You get put in a squad, and then when your Sergeant gets killed his 2IC takes over and the person who is the second-most-senior becomes his 2IC, so it's not necessarily a matter of having no room for advancement, it just happens slower than in an organization where the turnover is higher. As for librarians, there are multiple levels for them to aspire to, they start out as little more than magical book-keepers, and then get put in charge of progressively more important books as they grow in power and experience. I assume chaplains and techmarines also have their own chains of command they can ascend, you just don't hear about it because it doesn't involve murderfucking aliens or heretics with big fuck-off gunswords. 

And Medeis totally could have been Chief Librarian, even if Steve joined up first. Things normally go by seniority, I assume, but the thing about the chain of command, is that those higher up can dictate downward, so if the Chapter Master says "Fuck you steve, you numpty motherfucker, you're not in charge of the library no more" obviously someone has to be. Just because something usually happens, doesn't mean it always happens, that's something I've very recently been coming to terms with.