Even strong and careful skiers wipe out occasionally. A helmet is the difference between shaking it off and brain injury.
This correlates very well with the increase of helmet usage.
The trend is stable/slight increase in the past 5 years.
For the head injuries, it reduced from 2/1000 skier to about 1/1000 skier/year. It follows the general trend of the number of accidents per skier/year.
Looking at the material and the quality of the slopes in the past 40 years, they are definitely a big driver of this change, they improved a lot!
You can be as cautious as you want and never make a mistake, but if someone else skies into you you could still smash your head very hard. That's why you should wear a helmet. Not because of your own capabilities, but because of the capabilities of the people around you and the risk just being too high.
I wouldn't wear a helmet while cycling though. It's quite safe in The Netherlands.