IF someone was hit by a bus or car and their injuries are no way can be repaired and they are going to be in a stage where they can't really care for themselves at all in anyways possible way. Then why keep them to suffer?
Why would G-d keep them still alive suffering? If they should have died then why didn't it happen in the car crash?
Lacking an answer to that question, does not mean we should draw conclusions as to G-d wanting them to stay alive, or not wanting them to stay alive.
Here are 3 possible reasons why G-d may want that to happen.
It could be that them staying alive will have some affect on some relative, they get the chance to say their last goodbyes.
Or maybe because G-d knew that this person would be happier being put to death quietly in a hospital, than suddenly in a car crash.
Or maybe it is because another person in the car crash would have been too traumatised were he/she to find that person dead there.
Often when G-d does something, one may ask if it is a reward, a punishment, a test. Your example is really interesting, because it does not fit in there. But one can ask "how is it for the best".. and I can see those possibilities.
But guessing at the mind/will of G-d (beyond what he as told us) and drawing conclusions is a dangerous business.
(when I say "what he has told us". I speak as a believing jew, what he has told us in the torah)