Points well-taken, TheBoost. I don't deny that the social commentary aspect of the films were of secondary concern to Romero, but even eliminating that, we're still talking about a decade between Night and Dawn, and both films show the beginning of a zombie apocalypse. I don't think that this can be ignored or brushed aside as just as product of circumstances or the result of some need to start every zombie film with the outbreak. Romero was more than capable in 1978 of setting his film in 1968 if he intended Dawn to be a direct sequel to Night.
Instead it carries absolutely nothing over from the previous film other than "people attempting to survive a zombie apocalypse". This is where I have trouble accepting this long-held assumption that they depict the same zombie apocalypse. There's zero evidence within the films themselves that this is the same event, but plenty of evidence that they are not, specifically the large gaps in time, the lack of any recurring characters, or even one line of dialogue tying the events of one film to another. There is none. Believing that Dawn is a true sequel - and not just another film in a zombie series - requires far greater leaps in logic and assumption. I've heard the same thing from other fans. "Romero probably..." followed by some sort of speculation as to why the films don't appear directly linked but must be just because people have always assumed that they are. I mean, I've actually heard people try to explain with a straight face how Diary of the Dead, which uses hand-held digital cameras as a device within the film itself, is occurring simultaneously with Night of the Living Dead. The simplest answer, given the evidence, is that the Dead series is made up of standalone zombie films.
And going back for a minute to film franchises, I think you're forgetting The Planet of the Apes spawned four direct sequels, all made in the 70s, that are directly linked as one giant narrative. In fact, just a little bit of research will reveal the concept of film franchises stretching back to the beginning of film itself.