I use smartphone as camera. The single most important reason to upgrade smartphone for me is better camera because all the photos you take most likely going to live forever, probably spanning multiple generations. I think there is still lot of juice left in this area. If someone came out with new AI camera that started producing amazing super resolution wide lens images with professional colors, night vision, infrared, UV photography etc - that would be a huge deal. Battery is next most important issue especially when you are travelling and taking pics all day. If someone cracked battery issue or made this more power-efficient that would be huge case for buying a new phone. Headphone jack, non-slipper surfaces, not having to put ugly case on it - is 3rd most important reason to buy new phone. Peripherals, ability to use smartphone for other devices etc is probably the 4th.
But you see the problem... Folks have been rapidly increasing prices while giving nothing that customer wants.