Is iOS navigation bar placement at the top of screen a design flaw?

I always was wondered why the iOS navigation bar is placed at the top of the screen.

Navigation is a crucial of using an application, and some studies:

1. http://www.uxmatters.com/mt/archives/2013/02/how-do-users-really-hold-mobile-devices.php



2. http://scotthurff.com/posts/how-to-design-for-thumbs-in-the-era-of-huge-screens

show us that the least used part of the screen is the top part, because of how people holds their cell phones.

Android uses a back button located at the bottom of the screen:

that is conveniently placed and useful when you are using apps like the browser, when you have to go back several times: Google something, open the link read it, isn't what user was looking for, hit the back button, open another link...

Is that a design flaw?

Should the navigation be placed where the user can use it better (at the bottom)?

iOS reachability seems like a partial solution, because they are generating another problem: Half of the screen becomes unusable when you activate it.