20 years ago (pre-law), I would have expected that insurers would have treated helmet and non-helmet wearers equally. Today is possibly different as the question will always be asked if one could prove whether wearing a helmet would have prevented the injury. Insurers I also expect would take the view of yes, unless it could be proven otherwise. There would always be doubt and maybe be difficult to prove a helmet would not have reduced or eliminated a head injury.
If insurers chose to treated non-helmet wearers and helmet wearers equally should the law change, I expect that we will all pay for the risk through higher premiums.
The only solution for not increasing premiums for all, would be for the non-helmet wearer to accept the risk and liability of ones actions. This is unlikely to ever occur in the world we currently live as (legally) one seems to be able to reduce or avoid responsibility for ones own actions.