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While (afaik) seatbelt research is pretty definitive a very closely related area, car seat research, is plagued with confirmation bias, FUD, and a lack of interest in exploring alternatives (car seat makers are the ones who fund the crash tests)[0]. It’s crazy to go to hyper modern cities that don’t put children in car seats (Seoul, for one) while in the US you’d hear shrieks of horror if your kindergartener got into a car without one. So who benefits from car seat laws? It seems like primarily car seat manufacturers.
Back to seatbelts, I don’t know why someone would choose not to wear one, but if they have a reason that they value more than the safety a seatbelt provides, ultimately it affects them and their family far more than it would ever affect me. People need agency over their own lives. Not wearing a seatbelt seems like a minor inconvenience to most people, may impede others.
New Hampshire doesn’t require adults to wear seatbelts and doesn’t have as many per capita motor vehicle fatalities as neighboring states. In fact, it has the second lowest rate of deaths per 100M miles traveled in the country (obviously very different from large urban areas). So an assumption about seat belt laws equaling fewer vehicle deaths may not even be a valid assumption.
0 - https://freakonomics.com/podcast/how-much-do-we-really-care-...
If you're talking to someone who already thinks top-down regulation can be justified by saving sufficiently many lives, you don't need an argument like this.