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.
Paramedic here! They both serve different purposes. CPR keeps blood circulating to keep one’s brain and other important organs alive. An AED will detect the activity of someone’s heart and if it is a specific rhythm it will shock it. Cardiac arrest isn’t always a flat line, it can be the heart quivering or ineffectively pumping. This shock stops the heart briefly and then hopefully their heart will return to a normal rhythm.
In the simplest terms I can think of, someone in cardiac arrest needs both. Without CPR their brain will die. Without an AED they are less likely to come out of cardiac arrest and will just remain dead.