Ummm, what planet are you living on? Pretty much every post-apocalyptic book, movie or series evolves from the man-vs-nature struggle to survive to the man-vs-man fight for dominance. That's human nature and human history. And that's the way it has to be to keep the story interesting. Zombies are not all that great an enemy. They're slow, stupid and relatively easily managed once the survivors gain a little savvy in how to protect themselves. The story has to move on and pit the survivors against a more cunning and ruthless enemy: other survivors.
Christ, if TWD had ever truly been about man-vs-zombie and remained that way for more than a couple of episodes it would have run out steam and been cancelled. It has to be this way. In fact this is the original concept. This is based on a comic book series in which humans are the biggest threat, the entire series is based around human bad guys, the zombies are just window dressing. The issue isn't that it's become man-vs-man, it's that they're doing it badly. The bad guys are cartoon characters and the story lines go nowhere. That's what's killing the series.