IIRC backspace-to-go-back was not killed because of instant search, it was killed because users kept going back when they didn't mean to, because they thought they were typing in a text box and weren't.
But hey, cmd+q to quit immediately, with "warn before quit" off by default is a great idea. No need to look at that one or make it possible to change the key bind. Never mind that it's one key away from cmd+tab.
This is the kind of stuff you get when you make decisions based on A/B testing.
I probably could have been clearer in my original comment, my apologies.