I was into motorcycling for 25 years. Helmets are required all across Canadian provinces.
I used to subscribe to a motorcycle magazine. It often reported the stats as mandatory helmets were lifted or deployed across US states. These stats gave no doubt helmets save lives. Yet . . . . .
One thing that was surprising that the magazine often cited studies between the different kinds of helmets. Full-face helmets were marginally better than open-face helmets which were marginally better than the DOT-approved "lids" worn by riders who want to appear they are not wearing a helmet. Kind of counter-intuitive. But sometimes studies show these kinds of things. It's best just to put some protection on heads and not argue too much about marginal results.
I had a full-face for highway riding and a lid for town.
But no-helmet advocates will argue that these studies are a bunch of hokey-pokey. They know more than the people who are crafting legislation for a good societal balance.