There are two tv shows that have lasted much longer than I expected them to: The Walking Dead and Falling Skies.
They’re really the same story, with different window-dressing: the struggle for survival in the face of an inexplicable cataclysm. The cataclysm itself isn’t the point, but rather how the characters cope with it.
I have no trouble accepting a globe-spanning, civilization-ending catastrophe. (That sort of thing happens all the time in science fiction. A few authors have gone really big, and destroyed the entire universe.)
But the notion that a plucky band of survivors could sustain any kind of organized resistance is too much. War – against space aliens, against zombies – is as much an industrial effort as a military one, but there’s no industry without civilization.
Where are these plucky bands of survivors getting their weapons & ammunition? Their medicine? Their food? Scavenging is a short-term strategy, good for one season, maybe two. What happens after that?