"I don't write for the popular press," the writer says. "I write for posterity!"
(And then strikes a pose, hand on heart, black beret cocked at a rakish angle to highlight her artistic features.)
I never figured out how this one was supposed to work. Though unread in her time, the writer's works would magically manifest in the canon a hundred years hence, blipped there by... time travelers? literary-minded aliens? antiquarians who found her complete oeuvre on an ancient data stick?
And yet this idea seems perfectly reasonable to anyone who's completed a high school English course. The stuff in the canon, the stuff that's supposed to be the Glories of the Ages, is nothing like what we enjoy reading now. Shakespeare had a fine way with words if you can read Elizabethan English, but his plots are crap. Austen had a fine way with plots, but her style is dry. Dickens is purple. Poe is purpler. Gilgamesh is just plain boring. The entirety of the English canon contains fewer than a score of books that would be walloping good reads to the average reader of today. The point is that although these books are written in outmoded styles, some element of their stories, writing, or historical significance makes them worth reading past their sell-by date; but it's an easy point to miss. People can be forgiven for thinking that one of the defining elements of a Good Book is that it's written unlike anything they'd want to spend an airplane trip with.
And yet all of the Good Books were popular in their day. Dracula and Frankenstein were cheap trash. Pride and Prejudice was a good light read. Dickens was the first modern literary superstar, not in spite of but because his early novels were sentimental garbage. Shakespeare's works were The Sopranos; Chaucer's were Desperate Housewives. Even the Good Books that weren't popular successes were literary successes. These works were passed down to later generations because people liked them. Ordinary people, the Waldenbooks shoppers, the library-reading-group-goers, and the airline travelers of their day.
So if you want your works to live on for the ages, you have to write for those same people. (And they are the same people. Folks is folks throughout the ages; there has never been a Golden Age of Literary Audiences.) You don't have to write lowest-common-denominator trash. You do have to write books that contain what current audiences are looking for--sharply defined characters, fresh takes on the world, and it never hurts to be witty--as well as containing all the weightier stuff you're trying to impart to Future Generations. You have a hundred genres and styles to choose from, so you're not penned into the Deep Literary Mode or the Sentimental Romance Mode or the Action-Packed Thriller Mode or whatever else you think the world is herding you toward; but you are penned into writing a book that modern audiences will want to read.
Good luck. Writing a book that will last through the ages means writing not only a popular book, but the most popular of the popular books. You're trying to win the ultimate literary popularity contest. It's a grand goal and a hard one; so take my well-wishes and all the good advice you've ever been given, and go into it knowing what you're really trying to do.