Honestly, it doesn't make a whole lot of sense to me why anyone on that ship would get promoted. There's nowhere to go. No one is leaving that ship for a very, very long time. If you kept giving promotions to everyone who deserved it*, how many years would it take until it was a ship crewed almost entirely by Lt. Commanders?**
Yep, someone has to do the grunt work.
Who do you order to do the grunt work if everyone is of equal rank?