It's a tradeoff. There are already keyboard shortcuts for 'previous page' and there will be extension support to re-add 'backspace as previous page', it just won't be on by default.
If you were a drugstore and had a drug that was functionally identical to aspirin but caused day-long projectile vomiting in 99.96% of people who took it, would you think it was a great idea to hide the aspirin in the back and give the drug that is almost guaranteed to cause harm to someone as the default?
It's the first thing I'm always enabling on every new Firefox installation since they disabled it by default several years ago.
And with sufficiently large user base, even 0.04% means "lots of people being pissed off now".