Woah... Whilst you may have a point with your second bit (but surely, by the same logic, people are less likely to drive carefully in and around cars with air bags, which I can't really see happening) I completely fail to see how cracking your head on the pavement after falling off the bike is just as dangerous as landing with a helmet.
The risk compensation factor is well established. Studies have shown given compulsory seatbelts, ABS systems and airbags, drivers will drive more dangerously because they feel safer. As for your second point, read the disclaimer on the box. Helmets are not designed for collisions at greater than walking pace.
I know someone who was on a cycling trip somewhere exotic (I lose track of where) who had a bad fall at speed and landed on her head. Since then she's had all kinds of brain problems. I believe she's mostly recovered now (as much as you can do from a bad head injury), but the doctors were in no doubt that had she not been wearing a helmet then she'd have died.
I commuted a twenty mile round trip in central London for ten years by bike. I had numerous scrapes and an average of one serious 'off' a year. Not once did I hit my head. Anecdotal evidence. Not useful at all. Population level studies, however, show no benefit.
I remember a demonstration of the effectiveness of bicycle helmets which said that, when cycling at speed, the skull is like a bowl and the brain is like porridge. When dropped directly onto tarmac clearly the bowl breaks and the porridge goes everywhere, but when the bowl is put inside a cycling helmet then the helmet absorbs all the shock.
It's not big or strong enough to absorb anything more than a tiny fraction of the shock. I have seen a demonstration that shows that the increased effective diameter of the head due to helmet wearing can cause increased rotational forces inside the brain during an accident, increasing the chances of the brain turning into soup, albeit only fractionally.
If there were no benefits to wearing helmets, why is it compulsory (although, shamefully in my opinion, only in recent years) to wear them in the Tour de France?
Lewis Hamilton has to wear a helmet and
a full asbestos body suit when he drives competitively. That doesn't mean I need to wear one when I drive down to Cardiff on the A470, does it?