There are a number of detail design details and human factors at play here. The amount a human body will swing about inside a car will depend on seat design somewhat, and dynamics of impact. Say the head moves 3 inches in a side impact with similar seat design in most cars. In the big heavy German saloon cars I have sat in my head would be an inch into the side panel. In a panda it won't hit anything. Guess which one most needs side impact bags, and which one has them because it is a defacto standard. While the bag itself is clearly an important instrument not to be tampered with, it is a nonsense that they can't be refitted or have trim around them easily replaced and that is a symptom of bad industrial design... Unfortunately such cosmetic and optional driven rubbish is everywhere in our consumer society.
What if the human head only moves one inch or if it moves 10 inches?
Don’t make things up to suit your views please.
The idea that a head will only move three inches in the event of a side impact is laughable. A head will move more than 3 inches to the side in a head on impact...