Helmets may not prevent death in 60mph man vs tree encounters. Certainly can’t hurt though. Where helmets do come into unquestioned utility is less dramatic head trauma. As an intern a long time ago, I took care of a beautiful 25 yo woman who fell while bike riding in Catalina Island. Not a high speed event. Just lost her balance and tipped over. No helmet. Head struck ground in the worst possible place, right over the temple. Small skull fracture there ruptured the middle meningeal artery (which runs right under the temple part of the skull) leading to a massive subdural hematoma and eventually brain death. When it came to be my turn to have kids, they never got on bikes without helmets.
Your example adds a horrible sense of tragedy (to the discussion) that need never have happened. On it's face, bicycle riding seems tame and safe... life can be so damned fragile.
Having said that, a helmet hangs from the handlebars of my single gear coaster bike, as I await the pending spring thaw. I no longer aspire to the idea that I am “physically” immortal.
Flesh- meet asphalt!