I suppose my following point has been made but I'm bad at searching to see how it fares... So here it is...

Certainly helmets improve safety to some degree. It seems that the relevant item is that the improvement is such that it is relevant to cycling. Is there a determined threshold here? Did someone test or conclude that beyond a certain point that behavior should be modified?

For instance, helmet wearing for car drivers, walkers, bathroom users, or shower users, would certainly save thousands of lives, probably billions$. Certainly older people walking in possibly icy winter conditions would be much more likely to hit their heads. Same with older people driving -- or maybe first-year drivers: for some demographics crashes become more likely in cars or anywhere else. Why aren't people pushing for those demographics to also increase their safety? What is the threshold at which we encourage people to armor-up?

Why would we even need a threshold? If helmets reduced injury in even one person in one unpopular activity that hardly anyone does, shouldn't that activity still encourage helmets?

Certainly for very popular activities a tiny amount of safety improvement could still equal huge injury reduction. So what if a miniscule number of people hit their heads while walking? If everyone wore a helmet when walking then perhaps thousands of head injuries would be avoided -- yielding huge improvement in quality of life and money savings. The helmet industry and insurance and healthcare industries all alike would naturally want everyone at all times to wear a helmet.

Next, helmet effectiveness must be vastly greater the SLOWER the activity's potential impact. Sometimes biking and motorcycling has too much energy in crashes for helmets to be relevant. Walkers would almost always benefit from a helmet in a head-strike.

Next, is all ice-skating pushing just as much for helmets? Headstrikes are probably very common there. Like in ice rinks with rental skates. But really even among trained skaters the 'feet out, head strikes' scenario is common given the slippery physics of ice. Again, speeds are generally low: the typical ice concussion happens at zero speed with the "just standing there and feet went out." Helmets would give near total protection.