Late reply but to specify, the crumple zones dissipating energy to protect the occupants, but in part the situation you’re describing airbags do a great job at preventing people from hitting the steering wheel / walls.

A very very advanced harness system might compensate a little for a lack of crumple zones during a very rapid deceleration collision. The issue isn’t so much as stopping someone from but being thrown around in the car, seat belts do that, but nothing can stop one’s internal organs from doing the same thing inside their body. So when a body stops during a rapid deceleration, internal organs still try to move. This movement tears everything, most notably one’s aorta and a torn aorta means death with no possible chance of survival.

A small tear in one’s aorta and one may survive long enough for emergency services to show up, a bad one and they will have bleed out before a 911 call taker has time to answer a call for help.