'Back with backspace' was changed to CMD+left-arrow assumedly because a simple backspace can change the page unexpectedly for someone who thinks they are in a text field.
Back with backspace!
On top of that I actually have no "CMD" key on my keyboard, I have a "Ctrl" key which I assume is the same as "CMD" (I also have a key with the Windows logo which I had assumed it was the CMD key, I was wrong). KISS has gone out of the window long ago.
The outlook Web app on the other hand sometimes blocks backspace from deleting a character, presumably to stop you inadvertently jumping back from inside a text field. This is only "on" when inside a text field in the first place, so if MS could do it, I don't see why Google's better engineers couldn't.