Imagine that your brakes were so good that your car was completely immovable. If someone hit you from behind, it would crunch your car but you, personally, would not be affected.
Now imagine that your car weighs only 100 pounds with no brakes, and someone hits you from behind. Your car will accelerate nearly instantaneously to the speed of the vehicle that hit you, incurring very high G forces, in all likelihood breaking your neck.
I think your model of energy transfer needs some work. The energy that is not absorbed by the car itself on impact gets transferred to the objects in the vehicle. With either scenario, the same amount of energy is getting absorbed. The variable is time. If the vehicle is held stationary, all of that energy is absorbed immediately. If the vehicle can move, the same energy can be absorbed over a longer period of time.
an over-simplified example: Would you rather be tackled by an NFL linebacker while standing waist-deep in water or dried concrete?
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