I wouldn't say that Navigate on Autopilot hasn't seen improvement over the past few years. Even the original Autopilot has seen improvement over the past few years. There's a difference between improvement and new features. AP1 hasn't gotten new features in a very long time.
But there are striking differences in behavior in some cases. For example, with AP1 which people think of as stagnant, it will now slow down preemptively if a car in an adjacent lane swerves toward the lane line but doesn't cross it, leaving it in a position where it could slow down more of cut off but will get back up to speed of it becomes clear that the adjacent car isn't changing lanes. The old way would have been to maintain speed, take evasive action if another car cut it off, and the other driver would have been at fault for lane encroachment in case of an accident.
Navigate on Autopilot was a lot like FSD at intersections when it came out. It wasn't making outright dangerous maneuvers but was acting like an unsure driver. It would start changing lanes and abort, or feint to a side and stay in a lane, or fail to move when it should have. It was far from smooth. Over the years it improved gradually to the point that it's like night and day compared to the original release. It's not a question of new features but of a series of tiny improvements that add up to a lot.