I think the fact that iPhone SE maintains an old design is cunning segmentation of the product offering, with the sole goal of of luring users into the ecosystem, or encouraging them to buy into it at a higher level than they originally intended. "You mean I can get an iPhone for $400? And it's pratically as powerful as the high-end alternatives from either iPhone or Android? I'm sold. I want in. Going to do it. Oh wait, it only has that old display, and a low-res camera, and curved sides that are hard to hold on to? Oh well. I'll just get an iPhone 1X with the better camera and display. Because I'm already sold and want it and was going to do it."
For me, I don't want to spend the ridiculous amounts of cash on a high-end iPhone, and look judiciously at the iPhone SE every time a new revision comes out. Then I look at those terribly curved edges, the single camera, etc, and decide I'm going to just keep the last high-end model and eventually the newer high-end models, even though I don't want to spend the money.