People don't appreciate that:
1. It's much harder to create good industrial design than it is to copy it. When the Macbook Air was released, it was breathtaking. So were the first few iterations of the iPad and the iPhone. After the first big wins, it gets much harder to play the "smaller, faster, more storage and sleeker" game.
2. Technology matures. Many people rant that Apple's innovation around the iPhone and iPad has slowed. Of course it has, because all of the obvious things have been done over the last decade. It's like automobile technology -- once manufacturers figured out where all the basic components needed to go, they have cheerfully chugged along for decades with gradual improvement.
3. If you're the market leader, there is no value in going down-market. Apple does an outstanding job of maintaining margins without resorting to selling a bewildering array of phones at all price points in a desperate attempt to gain market share. Nobody wants a Samsung J3 or an LG K4. They're cheap pieces of junk that you only buy if you can't afford a decent phone.
4. Maintaining and developing iOS is a massive undertaking that Apple's competitors (with the exception of Google) don't have to undertake. We've seen Samsung's attempt at a third-party OS, and it was dismal.