I don't think that's totally true. For me the problem seems to come when the Showrunner either hasn't got a firm idea of where they're going, or the Network love the fact they've got a hit and want them to string it out as long as possible. 'Walking Dead' I think falls into the latter category...
Yeah, first series was absolutely brilliant. Second series not nearly so good and it got a bit formulaic. Third series - WTF? We're watching the fourth series more out of a sense of duty than anything else as it is the last one, and really hoping that somebody does actually kill Eve.
We're on the verge of giving up on The Wire. We're about five episodes into season 1 so it's still early days, but it's very boring. We also gave up on Westworld after Season 2. Season 1 was great, Season 2 was utter rubbish.