Part of the problem is that it's not necessarily quality going down, but the fact that quality itself is often defined by the amplitude of the shared experience. If thousands of people share the same art, then it's considered of great quality - even if it was just, say, a few white kids coopting some black music. When supply volume makes it fundamentally very hard to build such a shared experience, then it becomes difficult to recognise greatness in a (relatively) objective way.

It's the same for all art, as you say, starting from the figurative ones - the world has probably produced more professional painters in the last 150 years than in the whole history of the world, but among modern and contemporary works, almost nothing can really hold a candle to Michelangelo. Because there used to be one Sistine Chapel in the world, and now we're drowning in imagery every minute of our lives.