Count Lucanor
Thanks for that detailed description. So, in summary, you feel that the blend of mass cultural movements and technology has lead to a dropping of standards, at least in the mainstream.
I see this as more the result of economic rationalisation than postmodernism. It appears to me that, in all areas of human endeavour, there are two major tranches - that of experts and of the masses. Popular culture is cheapening all the time as makers become ever more formulaic and risk averse in an ever more competitive market with ever fewer second chances available after failure. For instance, imagine a wonderful piece and arrangement like Classical Gas being a #1 hit today
https://www.youtube.com/watch?v=mREi_Bb85Sk. It's unimaginable. Meanwhile artistic masters an virtuosos are ever more common in elite levels due to the broader availability of high quality instructional material.
Olympians break ever more records while the masses become fatter. Scientists know more than ever before as the masses embrace anti-science conspiracy theories. Medical knowledge and medicines are always improving while patient care - which so much relies on a doctor knowing a patient and the way their bodies usually operate - becomes rationalised and fragmented.
The bifurcated state of the arts appears to be reflective of the increasing schism appearing in many in societies, the natural effect of decades of neoliberalism, with resources "trickling down" while gushing upwards.