[AhmedFan2004]

I think you've drawn the wrong conclusions from a given set of outcomes. The viewing figures dropped for BB 8, 9 and 10 because of other reasons mainly. BB5 could have lasted for 5 months and people would have been glued to it. But a longer format will make a sub-par series drag on and ultimately become pointless. People will invest less time to a sub-par series.