I have an adult acquaintance whose life was saved by his bike helmet. He wasn't speeding, didn't have an encounter with a car, is a very experienced rider, etc. He just hit what was probably a very small piece of gravel on a smooth, flat bike trail. He went down, slid and his head hit a metal light pole, hard. The doctors said he likely would have been killed or seriously injured by his head hitting the pole, if he hadn't had on his helmet.
You cannot predict what could happen any time, anywhere, even on a smooth sidewalk iin front of your own familiar house. If that little head hits the concrete without any protection, you can't predict it will be "just a bump."
As for an accident where "the one NOT wearing his seatbelt had minor injurires," tell that to the parents of the three high school and college kids around here who all died in a recent car wreck who all died. None wore a seat belt. The driver, who did wear his belt, is alive. "And FYI this happens often" that people are killed by seat belts, you add. What police agency or statistical agency says that, how often does it happen, where, to what groups of drivers and passengers? Or is it something heard somewhere sometime?
I'd love to see the statistics on that, compared to stats on lives saved by seat belts.