The current cycling environment - of which mandatory helmet laws part of - reduce cycling participation to only athletic or risk-taking individuals. This means primarily young or middle-age males. In only one generation we have gone from most children cycling to school to almost none cycling to school.
Limiting cycling choices to the rest of society decreases overall health outcomes, increases congestion, and costs.
Cycling helmets do not (overall) make cycling safer - for various non-intuitive reasons that others have brought up in this thread. Hence few other countries have mandatory laws.
However on an individual level, it is apparent that a helmet may limit the severity of head trauma in a cycling accident. Which is the crux of the pro-law argument.
So this leads to the argument from me that - given we have 400 deaths per year on our roads - we should reduce all speed limits by 20%.
At an individual level in any single road accident, it is apparent that the lower speed limits will reduce the severity of injury to the individuals involved.
It is the same argument.