You NEED the fresh threat of zombie herds, and the certain amount of fright or at least overwhelming numbers, or the concept just doesn't work. Then it's just random post-appocolypse show, and whenever anyone dies, you stab them in the head and move on with your day...
Unfortunately, the comic set that "random post-apocolypse show" in motion, which is why the best zombie stories are short--as in the movies: there's a beginning, middle and end, because as you say, if it continues, its just a retread of the same survivor vs. zombie conflicts or survivor vs. survivor bickering/fights week after week, no matter the location, change of characters or bloodshed.
I believe the opposite is the reason the creators of the Resident Evil
videogames have been so wildly successful: they created a zombie outbreak, but added science and mysticism to the mix to expand the kind of threats the protagonists will face. The film versions did not execute this well, but the games opened the path to making the zombie plot something more complicated than what we see in TWD.