exactly. i was reading online about it, and the main purpose of the story is not about the actual event or the who is, just about the man and the boy and dealing with the mentality of surviving, not the actual surviving at that.
It's just all psychological. There are already many movies about nuclear holocaust, survival after the fact, zombie holocaust, but none really touch the mentality of it.
SPOILERS
I mean, this one really captures the fact the father just wants to die. simple as that. but can't bring upon himself to kill his son. He wants his son to do it but his son won't. Which is why every time they come upon trouble, he tries to convince his some to off himself instead of dealing with the problem or running away. He knows if his son can do it, then he can just finish himself and it will all be over.
Also, from what I understand, the movie changed the ending to make people "feel good" by adding the family taking the kid in. Why would a family, already burden with 2 kids of their own, take in another in such a world with no chance? Supposedly, the world's improvement rate was zero since life was dead (animals, plants, etc) which is why cannibalism was rampant.