The trouble with the first three films being set during the same zombie apocalypse is that each takes place roughly 10 years after the other and yet Dawn of the Dead clearly is set at the beginning of a zombie outbreak. If 10 years had passed, there would have been no chaotic televised arguments about what is happening. Things would've appeared much as they do in Day. Complete and total collapse of civilization from fade in. For Dawn of the Dead to be a direct sequel to Night, it would've needed to be a period-piece set no later than 1969, and yet everything about the film - from the costumes, to the sets, to the technology, and even characterizations - tells the audience that it is 1978.

What's more, as CP3S notes, and as I've always argued, each film offers a commentary on the decade in which it is set. Night comments on race relations in the 60s, Dawn on consumerism in the 70s, and Day on the military-industrial complex of the 80s. For that clear and admitted subtext to have relevance, the films must be spread out over a 20-year span of time.

I disagree that intelligent filmmakers were not debating continuity in the 1970s. It's not as if multi-part storytelling had yet to be invented. I totally reject the idea that George Romero thought it didn't matter that his films were each set in different decades. Quite to the contrary, it was of supreme importance.