Nevermind the sitcom being a dying beast, the real casualty of recent years is the sketch show. As far as I'm concerned, you have to go back to The Fast Show for a generally great sketch show. Nowadays sketch shows have become a kind of bland comedy muzak, clogging up the schedules with mediocre writing and acting. Take out the dreary sketch shows from BBC3 schedules and you'd be left with Two Pints Of Lager repeats and documentaries about people who only eat spaghetti hoops.
Okay, Mitchell and Webb was okay, but it was patchy. As for Catherine Tate and Little Britain, the phrase "overated tat" springs to mind. My suggestion would be to put a stop to all sketch shows until somebody can come up with something truly different and original.