"Abandon all hope, ye who enter here"

I give the author the benefit of the doubt, and don't think that he even attempted to picture a realistic post-apocalyptic world. As his is nothing of the kind.

There is no disaster imaginable that would kill the seeds in the ground, nearly all the plants, plants that can stay alive in such severe conditions and require only little sunlight, the fishes in the sea and in the lakes, all the birds in the sky, all the mammalians that can feed on grass and leaves... and yet leave human beings alive in such numbers, that the main characters in the book come across other humans so frequently, in grey abandoned environments.
It is also difficult to imagine how all the infrastructure of an organized society, like power plants that can sustain strikes of jumbo jets would permanently go out of function, while grocery stores and abandoned farm houses stay intact.
McCarthy's world is forever covered in grey inertia (many years after "whatever happened"), and there aren't going to be rains that would gradually wash the ashes away, winds that would start clearing the landscape or the skies, no green to one day raise from a fertile ground.

The world of this metaphorical story is therefore, and quite clearly I would add, more of an alternate/allegorical reality, like Dante's Inferno (or that of a persistent Biblical flood), but there is no Purgatory in McCarthy's version, and no Paradiso even in a glimpse of a dream. No poetic images of Beatrice here. The story forever stuck on the 9th Circle of Hell.

I don't know whether McCarthy was influenced by Dante, but his humans, who in their alternate unnatural world only feed on other human beings, hidden caches of food or rare rotten apples, turn into portraits of Dante's Count Ugolino who gnaws on the head of his rival Archbishop Ruggieri.

The characters of the alternate world don't either follow the logic and behaviour of our natural world. During great famines Ukraine, China, Ethiopia in the 20th century, that killed millions of people, cases of cannibalism were rare exceptions, not the prevalent occurrence that McCarthy portrays them into. His "bad people" have lost all their humanity, and have transformed into demons in empty human shells.
Compare McCarthy's world devoid of warm colours to the words of Mr Tsutomu Yamaguchi, "the luckiest or unluckiest man in the world", a rare survivor of both Hiroshima and Nagasaki, who 60 years later notes: "I have hope for the future." "I believe in love, in human beings"

There are no such redeeming qualities on offer in this book, no hope, no dreams. Those evolutionary wired qualities that during man's journey on earth have so many times pushed mankind through bottlenecks, and into new hope-filled beginnings.. The man and the boy never dream, never talk of a better tomorrow. And when, suffering from cold, severe starvation - and the man from something like pneumonia - they find their only glimpse of light, an abandoned bunker filled from floor to the roof with food, gasoline, water tanks to last them the whole winter and longer, they only stay for a few days despite the deadly dangers outside. The explanation McCarthy gives for them turning their backs on this temporary salvation, without even taking the time to heal themselves is as illogical as the rest of the book.
The man is afraid that "bad people" could any moment find the shelter that the man and the boy were the first humans to come across in the years since "whatever happened".
That moment an obsession (reaching south) starts to over shadow the prospects of his own survival or that of his son and the story starts losing what inner logic it had left.
The author is hell-bent to not allow even a small phoenix to be re-born from these ashes.

I don't see this book as the great story of hope or parental love or other such things the book is so lavishly praised to represent and I think this was one of the least deserving Pulitzer prize winners in history.
Of course our views are subjective, but comparing this book to some classics in which disaster struck people are portrayed (such as Albert Camus's sublime novel The Plague) I can't but recall Arthur Koestler's words from the last chapter of Book I in The Act of Creation:
"When he Reads Kierkekaard[substitute a famous author or a thinker], he is not moved by what he reads, he is moved by himself reading Kierkegaard - but he is blissfully unaware of it. His emotions do not derive from the object, but from extraneous sources associated with it; his satisfactions are pseudo-satisfactions, his triumphs self-delusions. He has never travelled in the belly of a whale; he has opted for the comforts of sterility against the pangs of creativity."