I started skiing a few months after I learned to walk, have been to most of the major mountains in North America, and used to ski 6 days/week in high school (slalom and giant slalom racing). I never wore anything but cotton as a base layer, except in the back country. Even racing (where we used ultra high speed tow ropes and could run the gates 45 times in a practice session), I never got very sweaty. Technical fabrics mostly matter if you're going to work your tail off getting up the mountain on foot, and then head down it at high speed, with the constant threat of getting stranded in the wilderness. If you're skiing bunny hills in-bounds, you're not going to need high tech synthetic textiles.
Also, when exactly did 90% of people start wearing helmets? I'm not that old, though I went a few years without skiing in the middle of the decade. In the early 2000s, it was mostly children wearing helmets. Heck, we weren't even required to wear helmets when racing slalom--just a chin guard. Now it seems truly unusual not to wear them. A lift operator even commented on how I wasn't wearing one last year ("oh, tough guy without a helmet, huh?"). Are head injuries really that common? Based on anecdotal experience, at least, it doesn't seem like it. Not one of my friends or teammates has ever hit their head. In fact, I've never even heard of such an injury by word of mouth. You'd think that if skiing were so dangerous, then some friend of a friend would have, at some point, had a head injury. I'm all about rational safety measures, but I can't help but suspect that helmet companies are just feeding the social amplification of risk in this case.