@mansr We 're on the last season, and I don't feel that the zombie aspect ever became irrelevant.
Most zombie stories are about how humans react to the zombie threat and its consequences, but zombies are the backdrop. This one is different because the zombies are also people and have agency, but it's the same thing: The focus is on how people act now that there are zombies.
Each season has different drivers and challenges, but zombies aren't just a gimmick and it's not just about Liv eating a new brain and McIver playing a new character each episode (although that would take me very far already, she's awesome doing it). None of the seasons would make sense if there weren't any zombies, the challenges and situations are unique. Maybe season 2 is the most "just a crime-solving drama with a brain-eating oracle", but not entirely.