The numbers you are referring to[1] do not mention lives saved but rather head injuries by transportation modes. They say nothing about seriousness of the injury. And I am pretty sure that when biking downhill at 70km/h chances are that a head injury will be more serious than the one i'd get by falling on the sidewalk while walking. I actually hate using a helmet while biking, and often go out without one when I am biking at a leisurely pace. But when I go out to train or to get to places fast, I prefer wearing the helmet.
I agree with your sentiment about the relative differences between broken bones and head injuries, but this statement isn't completely correct: if you're an adult, a broken bone almost always leaves behind some kind of problem, usually at a minimum the tendency to have arthritis at the point of fracture later on. Broken bones don't heal perfectly, just like significant cuts will always leave a scar in your skin.
Hopefully medical technology will figure out soon how to fix this, and make it so we can regenerate like those animals that regrow tails and limbs, but not yet.