PB - I think it is because art (and therefore culture) is cyclical. Periods such as the renaissance are little oasis within long dark ages. Witness the rise (and fall) of the Greeks, Romans, Venetians etc. There is a strong case to suggest that the fact our art has been in decline for some considerable time, that our culture is also approaching the hedonistic appocolypse that afflicts all great cultures?
PB - Re the chart. All predicted well in advance for those with an open mind.