[UserSince2005]

BB7 was the turning point, the series was very watchable but all the twists, extended duration and production influence really left no where else to go in future series. The exact moment the tide changed I would pin to them putting Nikki back in with the chance to win.
BB8 was actually watchable but following on from BB7 it could not compare to all of BB7s twist/entertainment, viewers switched off in their millions.
BB9 was the nail in the coffin, the housemates just ruined the entire series, it became completely uncool at schools and work to say you were watching big brother and so the writing was on the wall for the show.
I'm certain this is the case because BB10 was gold and it could not resuscitate the show.