It might be interesting to understand the problem from a UX point of view.
Up until phones like the Galaxy Note and iPhone 6 popularised the idea of a large display many forwent a case because the entire OS of their handset was usable with one hand. There was no interaction that was out of reach of your singular thumb.
When phones went larger both Apple and Google failed to rebuild their respective operating systems around this paradigm; they just stretched them, patched in temporary solutions like the fudge of ‘reachability’ and called it a day. It is a failure of software design that to this day there are any interactions on your device in portrait orientation that sit outside the bottom half of the screen. Because so many crucial interactions such as notifications, system toggles, search and in the case of iOS the ‘back’ shortcut and spare home screen icons that are in that top third we see more ‘finger-fu’ trying to reach them. The user lessens their grip and down the phone goes.
Now it might be easy to criticise so I will suggest an easy solution: remove the notification shade and control centre and add them in to otherwise unused space in the multitasking view. Notifications would stack at the bottom of the screen only to fan upwards on a press (like on the lockscreen!) and toggles would sit on a screen to the right of the top-most app. Problem solved.
From a hardware point of view it might help to use cheaper materials. Phone cases didn’t really start to become a thing until the industry adopted the design classic iPhone 4 chassis as its standard model and everything became a glass sandwich. Even with larger screen sizes I never once used a case on any Nokia Lumia device I ever owned because the backs were either interchangeable plastic or a magnesium alloy. The legendary Lumia 1020 had a wonderful metal body that sat in the hand beautifully. My old Lumia 620 is sat in a drawer in pristine condition because it I dinted the plastic back I could just buy a new one in a better colour.
From a commercial point of view if Apple were 100% confident that you didn’t need a case they wouldn’t sell their own and every model would come with 2 years worth of AppleCare as standard.
