Well, I’m not sure if it would be a matter of time, only. Probably our physique must evolve too, to be able to walk backwards (smoothly, for longer periods of time). The knee joint, and the ankles’ seem problematic here. I have walked backwards in the past; when I was in Greece and had to walk up a hill to get to my room (at night, slightly (not really slightly) intoxicated) I decided to try walking side ways (like a crab, trying to make the angle less steep) and backwards (in order to activate different muscle groups. It works, but it’s not smooth. Of course I wasn’t wearing goggles with images of what was behind me, but I peeked every few steps to see what was coming.