How about answering the point about helmet specifications and testing against what you are expecting them to do?
There is a whole regulatory and legal system in place to deal with that. Mainly because people kept making their personal judgements and getting it wrong resulting in death and injury.
There are lots of things people have done based on personal experience and then changed their views when the evidence has shown it to be misplaced. Smoking is a good example although there is still a hard core of people who assert smoking does not cause cancer and is good for you.
I use to wear a helmet on the assumption basis but then started to look into the research and stopped wearing one as a result. The fact that I can't know everything should not prevent me responding when I do. My diet is quite different to what it used to be, my exercise regime is too, I don't smoke and I don't wear a helmet. All based on research evidence. I am sure there are many other things I do that are not achieving what I think they are.
Your attitude seems to be that since you can't evaluate everything there is no need to evaluate anything.
I'm sure the tobacco lobby are the same about their belief that smoking is healthy.
The other comparison between head injuries and smoking is even simpler.......
You accept that there is a health problem, but then only deal with one small section of the problem.
You then dismiss any comment about the other sections as irrelevant, off topic, simply ignore the evidence or allow these sections to choose to continue to expose themselves to the risk.
Then finally you deem the 99% of related ill health that remains as acceptable.