I think there are two different issues at play here.

One, many severe injuries are focal. They may include a penetration or fracture or such. The injured area may take some time to heal but the rest of the brain is able to return to normal more quickly. Those injuries where the doctors sedate the patient to maintain a coma or coma like state, the objective is to reduce the brain's work load to allow it to heal and to reduce the chance of swelling. This reduced workload allows the brain to heal faster.

Second, many of us have diffuse injuries or more precisely diffuse axonal injuries where we have global needs to recover. Without the ability to have uninjured areas to carry the load while the injured areas heal, we continue to struggle along using the injured areas as best we can.

A third issue is whether we have other injuries like upper neck injuries that can exacerbate the diffuse injuries.

Think of it this way, if you broke your leg and had it cast and took time to let it heal by resting and using crutches it would heal in a timely manner. If the leg is not cast, one would need to take extreme extra care to stabilize it to allow it to heal. If this was not done, the leg would not heal properly and in a timely manner.

There are many permutations of complications to make this unable to be predicted. Best to not try to figure this out.