Only letting the viewer decide was not in Darabont's intentions. He had something to say and he said it as emphatically as possible. I know the "death of the author" is the authoritative critical approach today but it's not one size fits all. Ambiguity wasn't in his agenda.
Of course humanity survives. Humanity has to survive otherwise the "fuck you" to those who despair doesn't have weight. It has to be irrefutably proven wrong, no ambiguity about it. "Giving up is morally repugnant, but maybe not" would have been a pretty muddled message to end the movie on.