In either accident, the alternative outcome in not known and not
knowable. They died with the safety feature; they might have died
without. This doesn't argue for or against.

Ok, I can add one. My cousin was broadsided a long time ago, she was
not wearing a seatbelt and was able to jump into the passenger seat at
or near impact time. She believes she would be dead had she been
wearing her seat belt.

--Chris