Peter has been promoted during the course of the series--from Police Constable (PC) to Detective Constable (DC) and is now studying to be promoted to Detective Sergeant (DS). So he's not at the bottom of the totem pole, but he's not particularly high up, either. (Sahra's already a DS.) I genuinely do think that Peter being promoted and Nightingale retiring from policing to travel and teach will be good for both of them, and good for the Folly and the demimonde as well. It gives Peter room to grow, it continues the professionalization of the Folly, and it gives Nightingale a chance to do something he actually wants to do rather than a duty he fell into because there was nothing left. And, for the series, it gives us more paths to take. I can totally see novellas with Nightingale off touring the world, or him bringing something back for Peter to see and learn about.