Game of Thrones, The Wire, The Sopranos, Breaking Bad. All shows that have introduced compelling characters in later seasons.
I don't remember being sat watching the kids in S4 of The Wire and thinking "wish these would fuck off so we can get back to McNulty and co."
I'm curious, which characters do you think they've done a good job with that have been introduced since Season 3?
They weren't bringing in new main characters though. By and large they stayed the same. (Have never watched the Sopranos so can't comment on that.) They'd introduce new 'bad guys' like The Governor and Negan. Saul and Mike were season 2 I think, after that I'm not sure you could say a main character was introduced. People moaned about characters like Todd and Lydia. It happens to every show.
I would say they've done an okay job with Gabriel, Rosita, Abraham, Sasha and Eugene, although I wish he'd stop mumbling as it's somewhat ruining his heel turn. Tara, Tyreece, and The Alexandria & Hilltop lot were done poorly and Ezekiel is hit & miss. Michonne, The Governor, Negan, Dwight, Simon and the return of Morgan have been very good.