ego post comment the degree of effort audiences will apply to "something non essential like aesthetic nourishment." This leads to the knock heard for so long about jazz: you have to work too hard to 'get' it.
It's sometimes framed as "accessibility,' a word that either puts up walls or acknowledges that walls exist. The passage through a maze can be accessible. High mountain gaps in Tibet can be accessible. In any case, some effort is involved.
It's tough to quantify the elements of Hard/Not Accessible. Lack of recognizable melody? Complex rhythms? Polytonalism? Too much fortissimo? Certainly 30's jazz seemed-and seems-Not Hard to a lot of people, but "free jazz" does. Some strains of jazz have become background for car chases and sex scenes, thereby falling off the scale completely. Some people have brought in hip hop, rock, disco, etc. to shift the balance.
It may be true that one person's "hard" music is another person's "easy," but sales and downloads show that most people's hard is not some else's easy. It's their hard. too. Otherwise, there'd be a more random statistical pattern.
In a way, this question has a kind of moral/puritan ethic undertone, like that in the "personal responsibility" or the "greatest generation- versus-the-boomers" debate. I.e., that audiences don't have
the "right stuff."
Have the expectations, energy and capacities (lack of attention span or other psychic shortfalls) of audiences changed over time or remained stable? Is there a smaller group of people willing to tackle challenging music? Are there simply so many more aesthetic choices that this change isn't measurable?