Where does the idea that music output is equally good in all ages comes from?
The fine art world, and the literary world know very well from historical experience of centuries that there are periods of huge masterpieces and lots of great creators, and periods of drought. Heck, the classical music world knows that too.
Why would this not hold in the pop music world?
Music is another activity that is historically influenced by period, trends, culture, etc, and all of these have their ups and downs.