I am out of my depth in most of this discussion, so forgive me for jumping on this one point. What is wrong with analysing art in terms of the values pertaining at the time of its creation, and how those differ from those of today?
This is almost essential so we
don't project today's ideas on old work. Understanding work in its time is a tool for grasping its meaning and importance. It's when we try inflict our current ideas onto it that things go awry.
For example, a valid analysis might (or might not) be that Arbus concerned herself with the fringes of humanity she ran into in New York. This might lead to a further discussion of whether she was trying to merely report what she saw, whether she saw her subjects as victims, or whether she was trying to evoke empathy.
Compare that to the review that launched this thread. Arbus is scolded for being "classist" and thus out so very out of date with the modern mind. The reviewer is imposing today on yesterday, thereby adding no real understanding of her work. (Keep in mind that in the postmodern world, nothing is really objectively true
except if someone or something is accused of being
something-ist. The accusation alone is sufficient for a conviction of the crime.
)
Of course you can listen to Bach motets as though they are instrumental pieces, and they will still be great art; but
Well, not really, because the motets support a choir singing. Their absence would be noted.
isn’t it wise to remember the reason for their creation, what the words mean, and the religious convictions in Bach’s environment?
Yes, of course. To understand Bach is to understand his writing in the context of post-Reformation Christianity and his committment to it.
Is it unreasonable to do a Marxist analysis of Starwars or Marvel comics, to discover the value system that underpins them?
In that case, you're taking an old economic theory and seeing if it helps you understand something new. What you're not doing is looking back at Marx and condeming him for not considering the economic conditions of 3CPO and his fellow robots.
Would it be wrong to do a study of Rubens’ nudes in the light of current views on fitness and health? (ok, there I am being ridiculous to make a point)
You're actually not. In Kimball's book, he gives examples of this kind of absurdity and worse that are actual serious positions taken by arts academics and critics.