I don't think you are correct on this one, iPhone 3 was released 2008 then 4 had completely different hardware design in 2010 then 5 came in like you said iterated on and polished with again noticeable design changes and then 6 had completely new design in 2014 so in a span of 6 years and 4 generations of product you had a new design for each generation, so no it is not odd for Apple to do good hardware design they just stoped because it is easier I assume changing one hardware design element will disrupt the whole supply chain so yes they became very conservative with their hardware designs.