As i said, in general, going 15km/h is not fast enough to have a serious fall on your own. Only from external factors can riding a bike be dangerous, like from a car for example. in The Netherlands almost all cycle paths are isolated from cars as much as possible. So in general (again, in general) it is safe enough to drive around without a helmet.

The data backs it up. Look at the deaths per capita in the Netherlands. You can see a steep rise with the introduction of e-bikes, but before that it was one of the lowest in the world. And that is saying something when it is the most cycling dense country of the world.