I saw on telly ages ago a thing they used to convince people seat belts were a good idea when the law changed to make it compulsory
It was a seat on a very short ramp that slide into a buffer giving a short sharp stop at 5mph, no-one, no mater how anti seat belt was prepared to try it out without the seat belt
They should make a machine like that for cyclists that threw you head first into something at 5mph
Say 70 kg driver car hits brick wall & takes 0.5 seconds to stop, Driver thrown forward on very slippy seat, max total force on arms against steering wheel and feet against floor/ pedals 308N say 68 lbf - I think I could do that, so usually don't wear a seatbelt when maneuvering my car at low speed not on the public highway.
No injury likely. Wouldn't want to try hold the 10000N or so the seat belt is designed for by not wearing a seatbelt at speed when might need to brake suddenly 'though.
Accidentally hitting a wall with a car at walking pace is a quite likely incident. So is rapid braking at 70mph - I've done both myself.
70kg cyclist projected straight at wall so head takes entire impact energy 172J. Helmet design limit <100J - foam compresses, & helmet thin cosmetic shell possibly cracks as foam no longer holds it in shape. Likely injury to neck.
No helmet, 6mm thick solid bone skull takes impact and survives intact. Very sore head. Blood everywhere, as cuts on head usually do. Likely injury to neck.
Being projected from a bike horizontally head first at walking pace is a quite unlikely incident - in fact I've never done it. Come off & landed on a shoulder a few times.
Having considered the facts, I can make my decisions about helmets & seatbelts.