The "classic" helmet was a padded leather job called in the day a "hairnet" or "skid lid". The joke in the amateur peloton was that we wore them so we could have an open casket funeral. The helmet with a compressible, collapsing foam liner was a huge step up. That foam does far more to protect your brain from the acceleration inside your skull than any hard shell will ever do. (I know. I did the impact test with the first generation Bell in 1977.)

Your unpadded hardshell hat might well get you the open casket funeral. If you can accept not being able to participate as witness to that gathering of friends and relatives, go for it.

Ben