Earlier quoted context omitted.
I mostly develop for Android devices and use iOS. I have literally never missed not having a "always-back" button. Where would I want it that a way to go back isn't baked into the flow? I mean half the Android apps out there have broken backstack implementations anyways so pressing back on Android will do anything from send you back one screen to set your hair on fire and everything in between so...
The most common scenario I have is switching from RSS reader into web browser for the full article and back. This is also about 50% of what I do on the phone that's not scrolling web pages vertically.
(Switches between your current app and last app for reference)