I'm not a zombie junkie, but like a few series, when it gets properly silly I tend to lose interest but most shows are like that...LOST to name one. I reckon they don't expect to as successful so don't plan for the long haul. I do like a zombie film tho, 28 days etc especially but when series run out of ideas they turn crap.
I liked Z Nation, prob coz it didn't take it's self seriously.
I've had a gripe with TWD since episode 1 tho... not a SINGLE character has mentioned the word Zombie. (as far as I know).