https://www.headway.org.uk/news-and...act,mortality, severe traumatic brain injury (Actually I suspect you haven't seen any evidence from neurosurgeons but merely lay persons' assumptions with a veneer of "I'm an expert". I'm happy to be proved wrong
https://bmjopen.bmj.com/content/9/9/e027845
https://www.boltburdonkemp.co.uk/our-insights/posts/cyclists-the-difference-a-helmet-makes/
https://road.cc/content/news/dutch-neurologists-call-cyclists-wear-helmets-286871
Dr Phil O’Halloran, Consultant Neurosurgeon at Beaumont Hospital, has said “You can’t ignore the numbers. Wearing a helmet won’t stop all serious head injury but it does reduce the risk of skull fracture.”
Of course there is even disagreement between neurosurgeons:-
https://road.cc/content/news/121280-cycle-helmets-save-lives-says-neurosurgeon-ongoing-helmet-row