I imagine what makes art "great" is popular acclaim?
In the short run, i.e. the contemporary art of an era evaluated in its own time, "great art" is art that is regarded as such by the art critics and art scholars. They have as much disdain for the tastes of the masses as any curmudgeon on this website. Popular acclaim only identifies art as
popular--oh yeah, and it means that the artist can make a living.
I'm more familiar with music than the visual arts, and you can certainly see this dichotomy in contemporary music. Popularity is often fleeting: "I'll be put in the back on the discount rack like another can of beans," to quote Billy Joel's introspective song, "The Entertainer." (How nicely ironic that he's still out in front.)
A few artists are recognized as "great" by the critics and scholars in their time, and this judgment is often borne out by enduring popularity: The Beatles, CSNY and Pink Floyd, for example, are still beloved in anglophone countries and elsewhere. "Dark Side of the Moon" would still be on the Billboard Top 100 if they hadn't made a special rule to disqualify it, just like the New York Times Book Review doesn't have to keep listing the Bible and the Boy Scout Manual as the two best-selling books every year.
But pick a week in the 1960s or 1980s at random (skip the 1970s, which produced most of what is now called "classic rock"), look up the top-sellers for that year, and see if you even recognize the titles.
Personally, I either like it, I really like it, or I don't like it...and that seems to have very little relation to what art critics or art historians think about any particular work.
In any art, you have to recognize the enormous difference between a work's ability to communicate with the people of its own era, versus speaking to the people of the future. Again, sticking to music because as a not-very-visually-oriented person painting is not my strong suit, some songs absolute tear at the heartstrings of the people who first hear it, and then don't relate to those who come after them at all. All the "protest", growing-up, and discovering-life songs of the Generation Gap in the late 1960s and early 70s: "Universal Soldier," "Woodstock," "Eve of Destruction," "Little Boxes on the Hillside," "Society's Child," "The Circle Game," "Both Sides Now," "Puff the Magic Dragon." We cried and/or marched to those songs, and to many of us War Babies and Baby Boomers they still bring a tear to our eye, but younger people just don't feel them, or even understand them.
Whereas songs about more general, eternal themes, like heartbreak, having fun, feeling bad, and love (of a person or a place!), are still on the radio. "Sweet Home Alabama" is full of references to the politics of the 1970s that no one gets any more, but it's also a sweet love song to Alabama, and in this era of globalization, to America. (Kid Rock is a Yankee from Michigan!)
Composers of music without words--symphonies, etudes, concertos, string quartets, etc.--don't have to worry about the topicality of their references. And indeed far more instrumental music from the past is still in the repertoire than vocal music. But they have to find some other way to connect with their audience or they just end up writing ditties that strike the public's fancy for a few years and then vanish into obscurity.
Just like "Rebel Rouser," "Baby Elephant Walk," "Walk Don't Run," and "Music Box Dancer." Those were enormous hits--do you folks even know them?
I haven't sold out yet to The Man, but the instant The Man's buying...poverty bites.
There's nothing wrong with creating art that speaks to your contemporaries, even if it won't speak to their grandchildren.
And on your Bach slayings, I can only say, that "plagiarism" as you described it, was prevalent in the Baroque era. Composers would often "sample" a melodic line or motif and either incorporate it into their own work, in which they would often put in the title "after ________ composer" or they would completely transcribe a work by another composer into a different format. In the case of Bach, he had several famous transcriptions from Vivaldi that he made into different translations, keyboard concerti and whatnot. Composers would also borrow themes from their own works if they liked them or they were popular. I would not call that plagiarism.
Today it's generally only lyrics that are copyrighted. It's very difficult to protect the instrumental portion of a song. You have to very nearly perform the whole thing exactly as scored to be sued.
And "modern art"... well, if people can make money selling crap to those willing to buy it... good luck to them.
Iconoclasm has been a very popular philosophy for the last few generations, so there has been an increase in the population of people eager to support artists who produce crap. But still, you can hardly dimiss all modern art as crap. Mikhail Chemiakin is so "modern" and iconoclastic that the Soviets locked him in a mental hospital, but we love his work and have it on our walls, and my wife can even explain it to me. Humboldt County, where we live (except for my extended stay on the other side of the country finding work) has the highest concentration of artists in the country so we go to a lot of art shows. Although some of their stuff is patently crap, much of it is astoundingly good, despite being "modern." And considerably more affordable than a Chemiakin.
