There's an awful lot of conversation about real world ranking systems, but look at the World of Trek. Riker could have been Captain in six years apparently, and Harry Kim stays an Ensign for seven. I don't think Riker's a particularly wonderful officer, and I don't think Harry Kim is a horrible one. It just all goes back to what I said originally - the writers assign rank on the main purpose of what rank fits best for what kind of story they want to tell. Not on any particular real world feasability.