We didn't really see in higher ranks at Helm and Navigation until the films. They seemed to top off at lieutenant. Sulu, Chekov and Uhura working in their TOS postions in the film is an anomaly not standard practice. Of course Chekov was Security chief in TMP and XO of the Reliant in TWOK before seemingly going back to navigation on the E. An odd career path.Oh I agree with your observation on importance but as all this got kicked off by having t0o many crew of Lt Cmd + the Captain to fit in Senior Rank Cabins one had to be less than Lt Cmd and Quartermaster just seems to make the most sence and fit in with most of what we have seen - Either that or as the role of Operations seems to be in chage of Helm and Navigation you might argue that role doesn't exist - we have never seen a Operations officer on 1701 or 1701-A but then again Helm and Navigator where usually higher ranking than Comms..... Is Security the one to get dropped down? The problem is QM is the only Officer listed on the Plans who is 2 levels above the next highest ranking in his department - so is the only one you could lower without having to move any body else......
Helm and Navigation are combined as Con officer in TNG. Operations was created for Data, but its more or less the Science Officer job ( exposition ), not helm or navigation. Data is also Second officer. which might account for his Lt Commander rank. Others on the bridge, Laforge, Worf and Yar, were lieutenants. So again, not very many Lt. Commanders at the start. The eight "slots" for Lt. Commanders might not normally include various bridge officers.