Leave "WWTBAM" off the list of failures of primetime shows. The reason it died was because of too many celebrity shows, and only one "regular" contestant show per week, which hurt continuity. I think the loss of the phone game also hurt as well, though I'm sure there are some here who will disagree (though the ratings did perk up again when they brought the phone game back).
Leave "Twenty-One" off the list of recent failed primetime shows as well. It didn't die a natural death, it was murdered. After receiving respectable ratings in its original timeslot, it was deemed a failure by NBC because it wasn't receiving WWTBAM-style ratings. It was moved to another time slot, where after an initial drop in ratings, it managed to hold its own and even improve. Then NBC wound up cancelling the show, even though it was getting higher ratings than any number of shows they didn't cancel.
Let's not forget "Greed", which was cancelled by Fox; they haven't had a single show perform as well since then in its Friday timeslot. "The Chair" and "Winning Lines" were lousy shows. "Weakest Link" was borderline at NBC when they cancelled it. Shows with gameshow-like competition elements like "Fear Factor" and "Survivor" are certainly doing well.
Most new shows fail within a season or two - that's just the way the business is. You wouldn't expect game shows to be any different, and they're not. There will always be more failures than successes. But based on the comments that came out from the networks at the time of WWTBAM's original success, programming executives don't like game shows, and will look for reasons not to program them. If a primetime sitcom fails, no one says that "primetime sitcoms are dead". When WWTBAM's ratings dropped, lots of people said that "primetime gameshows are dead". Then they bring WWTBAM back in a lousy timeslot, and it winds up getting some of ABC's best ratings for the season.
