"Unless you want to argue that I would have been just as fine hitting the ground at 25km/h without a helmet?"

Why not? Humans are pretty resilient. Maybe you wouldn't have, but I see no reason to dismiss so easily the possibility that you might have been fine.

Mentioning personal experience is fine, but you shouldn't make broad statements from it unless the experience actually justifies it.

Think of it like this: somewhere out there is a story that's the opposite of yours. Some guy wrecked his bike and hit his head without a helmet and was fine afterwards. He then concludes that helmets don't work. His reasoning is no different from yours, yet reaches the opposite conclusion. Clearly the reasoning must be flawed.