The good and bad of bottom nav
Bottom nav was a great idea when Apple first came out with it. Steve was laser-focused on one-handed usability. The bottom nav was designed to accommodate fast and convenient view switching where the mobile use case seemed to demand it.
Unfortunately, bottom nav is a hierarchy nightmare when used for an app's main info architecture. Product designers primarily familiar with iOS began to take the lower position as a given and put everything down there. As web apps became a viable mobile solution and the top navigation was carried over as part of simple responsive layouts, a new pattern started to take shape.
Enter Google's gradual shift to an experience-centered org ...
Top nav makes sense
Look at the Material Design guidelines for a well-developed perspective on top nav. They've recently added bottom nav for frequent switching scenarios, but it has a little different angle in MD.
Part of the logic behind MD's top nav solution, in addition to the obvious info hierarchy win, is the fact that action should guide you through an app's views. We don't have to force users to tap tabs and manually move through their workflow.
Taking that even further, Google has advocated an activity-centric feature model: regardless of the view or even the app, an "intent" will take the user where the app component they need.
The recent addition of bottom tabs in MD is not just in response to the iOS-minded product designers of the world. The use of bottom tabs can be used where manual rapid view switching is needed. Thise seems particularly good in a gesture-based view where you want to provide guideposts for left/right swiping behavior (though top nav can do the same).
The bottom nav perspective on physical usability has an issue: user's have made up their mind that it's not a big deal. How many 4" phones are making hot tech lists these days. The remaining small-format phones are mostly budget devices. So how many users are one-handed phoners now?
For one-handed iPhone 6+ users, should we develop a bottom-left nav? 😂
This is probably more realistic ...