What's the problem with the 20% warning? Plenty of time to come back with that (unless you have one of them iPhones with the defective battereis that shut down unexpectedly at random percentages, but that's another problem).
On android you just swipe down and see the top bar with the battery level, no need to put the app in the background.
20% battery warning is not a great place to be with a phone running at 90% brightness. It's not long before the 10% warning hits and then you have a more complex dialog box pop up with three options.
Also most of us iOS users are not flying with Brand new iPhone 7+ we are using two year old devices and on top of that you assume that a person is taking off with a full battery. I have taken off on my fourth flight of the day with 32% battery life and for that very reason I kept the drone within 100 ft range and just practiced various flight maneuvers. I might have gone out further if I had the ability to constantly monitor my phone battery.
Swiping down on an iDevice is a messy business. You can get the battery life as well as a screen full of notifications. Also its not always easy to swipe down or up without hitting one of the buttons on the DJI Go4 App.