Why The Walking Dead Can't Survive Without Rick Grimes
3. Carl Was Its Only Future
Rick may be the show, but there was once a possibility of a future without him in the form of his son, Carl. The story was his as much as Rick's, if not more: he was the entire reason Rick fought to survive and create a better world in the first place, and - while Chandler Riggs wasn't the best actor - Carl was the only character well-placed to take the story further in the event of Rick dying.
Of course, The Walking Dead being The Walking Dead, they killed the younger Grimes off in Season 8. Not only was this an invalidation of Rick's entire arc up until that point, but it also robbed the show of its clear future. Without Carl there isn't anyone to really pass the baton on to.
There are plenty of other characters, but none who could have the same sort of arc, and most have reached points in their journeys where there isn't all that much more to do with them, at least not as the main character. That's what Carl represented, and one of his reasons his death was such a huge mistake.