Your quote started out batting 1.000 in the first half: You're right Apple doesn't need a design change, nothing wrong with it per se. If I was to make any change it would be to find a way to make it less slippery with the rounded edges. And the bezel. Holy crap the bezel.
Then you completely struck out: There's no logic to your case and accessories argument. Cases for 5C, 5S, 6, 6+ aren't interchangeable. Every new phone has required a different case size. Not sure where you're getting your Android and WP info, but it's wrong. Cases and accessories are available for pretty much any phone from budget to flagship.
Jobs championed 3.5" as the right size. Cook said 4". Neither was right then, and it's still not right today. The right size is the one that each individual customer chooses for him/herself. It's okay to want a 4" phone. Everybody has preferences. There is no one right size.