When you wear a helmet while riding your bicycle in public, you send an unmistakable message to everyone that sees you -- and that message is that riding a bicycle is dangerous. That simply is not true, statistically, and broadcasting that falsehood is detrimental to the collective safety of cyclists, because it reduces cycling uptake. When people are led to believe that cycling is dangerous, and an activity suited only to daring young men and a few other crazies in lycra, they don't become cyclists. Or more likely, they stop riding the day they get their driver's license and never look back. And the only thing that has ever been shown to actually improve cyclist safety, is more cyclists. With more cyclists on the roads, other road users become accustomed to them, and they learn how to cooperate with them -- and that current lack of cooperation is almost-entirely the extant safety issue today.
If I only rode a bike on a bike path, out of the path of travel of cars, at an easy-to-moderate pace, with other competent riders, I might consider not wearing a helmet. I don't ride that way, and I don't want to. A large majority of the people I ride with don't ride that way, either. What you're talking about is a societal shift...and a fantasy. In the real world, and the places I enjoy riding, I'm going to stick with wearing a helmet any time I ride further than the end of my block, because doing so has proven to protect me from a more significant injury than if I hadn't been wearing one, multiple times.