Why do we no longer have a navigation officer on modern commercial flights?
Navigation has gotten much simpler over the years.  Initially, navigation would be done by a combination of dead reckoning, looking out the window for landmarks, and for night or long-distance flights, by celestial navigation (many old airplanes have a window at the navigator's station specifically to let the navigator see the stars).  This took quite a bit of time and effort, so the pilot couldn't do it and also fly the airplane.

In the 1920s, radio navigation was added to the navigator's toolkit, but finding bearings to beacons and calculating a location was still an entirely manual process.  Additionally, radio beacons didn't cover the entire world, so older navigation techniques were still needed for most flights.

In the 1950s, fully-automatic radio direction finding systems became common and navigation beacons were available across most of Europe and North America.  At this point, the workload involved in navigation dropped to the point that the pilot could also navigate: plot out the sequence of NDBs to follow ahead of time, then tune the radio and follow the navigation display.

Very recently, navigation has been integrated with the autopilot to the point that the pilot can punch in a sequence of waypoints and have the airplane fly the route entirely on its own.

And yes, even today if the electronic navigation system fails, the fallback is map, compass, and slide rule.