I think about things like, wouldn't they eventually run out of zombies? In the initial outbreak there were zombies everywhere from all the regular people, but if we assume they decay wouldn't those eventually get to a point of non-function? By S4 the survivors wouldn't run into zombies caught doing everyday things anymore like they did in S1. Wouldn't it get to be where the only zombies around are from the few survivors, when they eventually die? No more giant herds of zombies, just zombie Herschells and T-Dogs etc.
But then I decide it's a show about zombies and it's better not to look too deep into the logistics of the whole thing, haha.