This to me has always been one of the most depressing motifs in mainstream comics, whether in X-Men, KC, or that crappy "Fantastic Force" thing. Mankind is replaced or preserved by the superheroes because of massive disasters and breakdowns of the sort we have already started to taste. Disasters that nothing else could save mankind from, or could survive our extinction and assure civilization's continuation.
Y'know why that depresses me? Because there ARE NO SUPERHEROES.
So when the world dies...well, when you think of it as something now very difficult to slow down in the real world, the stories like this come off as a cruel taunt, if you really think about it as deeply as the creators want you to.(though maybe not to those conclusions)