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...
Navigating within apps is frustrating enough between apps that intentionally implement wonky custom stuff for no reason and cross platform apps that don’t obey conventions by nature, the last thing I want is those same apps mucking up otherwise consistent app switcher behavior.
(Switches between your current app and last app for reference)