I've never used backspace for nav intentionally, and it has caused annoying data loss for me a few times. It's never made sense to me why this behavior was ever added to browsers. The logical choice would have been the left arrow key (since there is a corresponding right arrow).
Without some sort of modifier key, this would come with the same set of problems, i.e. failing to focus a text field while using it to move the cursor would trigger navigation. It's also used to scroll horizontally, so it would break other functionality. Alt + LeftArrow already works.