Not wearing a helmet and scrum caps aren’t why rugby has less concussions so I’ve never understood the scrum cap debate for football. It had less concussions because of the laws of the game. You aren’t allowed to hit without wrapping in rugby. No throwing a shoulder into a ball carrier, no throwing your body at the feet of a ball carrier, no tackling above the shoulders (they are even looking to eliminate tackling above the torso soon) etc.. you have to hit with your shoulder and wrap your arms. If you don’t follow that, you get a penalty. Do it more than once and you can get a red card and are ejected. Rugby players are taught from the lowest levels of the sport how to tackle and how to protect your head going into a tackle and they practice it year after year, even the pros. This is why they have less concussions.
I actually got one concussion in rugby after 5 seasons. The one time I did, I was wearing a scrum cap. The other 4 years, I didn’t wear one.