"Of course, in those days when you wanted to ride your bike, you just jumped on it and off you went. If we'd had to dress up like quarterbacks every time we wanted to run to the store or a friend's house, we probably would have lost our taste for bicycling, too."
Yes. We all rode bikes everywhere when I was a kid, but I seldom see kids on bikes now. Bicycles weren't just for fun - they were our transportation to friends' houses, or really any location within a mile or two from home.
However, I don't think it is just the helmets (though that plays a part). It is the general trend of children seldom going outdoors, driven largely by helicopter parenting.
More generally - I approve of helmets for highway use, but think we would be better off without helmets for casual cycling on city streets with low speeds. For one thing, most people don't want to carry a bicycle helmet with them everywhere they go, and helmets mess up your hair too - which might seem silly but do you really expect office workers to put up with bad hair every day when they could just drive instead?