A couple of things.
One, you're naive if you think anything can't be hacked. Everything can be hacked, even the most secured systems in the world can be hacked with enough resources put behind it. The iPhone may be more secure if a random thief steals your phone, but if some one like the FBI or other government agencies want into your phone, doesn't matter what you use, they'll find a way in.
Second, not saying apple NEEDS to do anything, they're very successful in what they do. But imagine if you will a world where the iPhone 6S/6S Plus are the same iPhones you know and love with awesome apple services and integration, clean, fast, reliable performing ui, world leading apps, a9 chip, ultra fast ssd, and all metal design, but also have a smaller footprint because of smaller bezels and curved edges, is water proof, has a better low light camera, Apple Pay is available everywhere, even on old credit card terminals, has multi window, has a more customizable home screen, has a file system, has fast charging, and has a 3,600mah battery. Wouldn't those things make using the iPhone a much better expierence? And in this world, the discussion would again be how the iPhone was ahead of the competition instead of how they're all pretty much the same and it more user preference than anything.
Again, the iPhone is fantastic the way it is, but it could be so much better if Apple had been more aggressive. And this is with current off the shelf technology available to most manufacturers, who knows what future tech apple has in its labs.