The argument doesn't overlook this. The stimulus explicitly states that "all" contemporary artists hold this belief.

The argument isn't about whether people want to go out and appreciate art. It's about whether contemporary art enables more aesthetic fulfillment than people would otherwise have. Even if most people never bother visiting museums, that wouldn't affect the author's reasoning. The author is making a claim about what could fulfill people, not about whether people are motivated to seek out that fulfillment.

The argument never takes a position on what determines the "value" of an artwork. It's focused on whether contemporary art makes people more aesthetically fulfilled than they would be without it. That's a claim about the audience's experience, not about the artwork's worth. Even if an artwork's value were completely independent of human appreciation, the author's argument about people's fulfillment would be unaffected.

This points to the gap the author overlooks. The author reasons that because there are already enough great artworks to satisfy any taste, contemporary art isn't making anyone more fulfilled. But this reasoning works only if people can actually access all that existing art. (D) raises the possibility that many people's access to the great majority of existing artworks is severely restricted. For those people, a contemporary artist's work that is accessible to them might be the only way they encounter art that suits their taste. If you remove that contemporary artist's work, then those people would lose fulfillment they can't replace, because the supposed substitutes are locked in private collections or displayed in museums halfway around the world.

Notice that (D) only needs to show that at least one contemporary artist's work does this for many people. That's enough, because the author's conclusion claims that all contemporary artists are mistaken in believing their works enable many people to feel more fulfilled. If even one artist isn't mistaken, the conclusion is wrong.

Pay attention to the word "affects." (E) is saying the author assumes that the existence of lots of great art changes how much fulfillment you get from a contemporary artwork. Like, a contemporary painting would somehow become less beautiful to you just because the Mona Lisa exists somewhere. That's not what the author is arguing.

The author isn't saying existing art changes your experience of contemporary art. The author is saying existing art could replace contemporary art. The argument is: "You could get the same fulfillment from art that already exists, so you don't need contemporary art to feel fulfilled." That's a claim about whether substitutes are available, not about one artwork changing the experience of another. And the real problem with that reasoning, as (D) identifies, is that those supposed substitutes might not be accessible to you.