To an extent, some brain damage is reversible, especially at a younger age. Brain plasticity at a young age lets certain parts of the brain act for inactive, or disabled parts of the brain.
There is a documentary of a young girl who used to suffer from grand mal seizures. She went through a Lobotomy surgery that essentially removed the entire left hemisphere of her brain. After only a few years the right side of her brain was able to take control of the parts of her body and functions that the left side of her brain had originally been in charge of. But, of course, she was unable to COMPLETELY have the damage done to her brain reversed.
The main problem is that the older you get, the less plasticity your brain has, making it harder for brain damage to be reversed.
There are actually two(!) little girls in my town who have had that surgery. Although you are right that they will probably never be able to completely regain their old selves, they are doing quite well. It really is quite amazing.