At some point you just have to accept the evidence.
The main question to ask is this: why exactly do mandatory helmet laws reduce bicycle usage, and what needs to be done to address it?
Is it that helmets are too expensive? (They aren't.)
Is it that they are too difficult to put on and take off? (They aren't.)
Is it that they are too uncomfortable? (Maybe, but not any more so than seat belts, and yet most people aren't arguing against seat belt laws.)
Essentially, I'm not against accepting the validity of the evidence from these studies. I'm simply against the mentality of "mandatory helmet laws reduce bicycle usage, therefore we must not have mandatory helmet laws." That conclusion is too simplistic. The topic requires further study.
I wouldn't assume that. As far as I can remember, this is one of the main reasons cited for the reduction in cycling. Small things can change a persons behaviour significantly.
There is also potentially (depending on which studies you give credence) the danger of risk compensation causing helmeted cyclists to cycle more dangerously and automobile drivers to pass more closely to helmeted cyclists.
Even if the benefits are "obvious", the large majority of people aren't self-improvement-focused supermen with significant reserves of willpower at their disposal. Even the smallest increase in the necessary "activation energy" (comfort, expense, convenience; whatever) necessary for a task is going to put a large chunk of the distribution on the "fuck it, I'll just drive" side of things.
This is why "engagement" and "user experience" are multibillion dollar industries. Compare abstinence-only education, condom usage, and (kinda, if you squint a bit) drug prohibition.