A few people have linked to an article in The Guardian about genre ghettos and the Booker Prize. There are plenty of things to argue with in the article, and I'm only going to attack one right now, because it's something I've heard from all sorts of people over the years (and certainly not just people trying to defend the honor of genre writers).
To say, "If Dickens were alive today, he would write soap operas," is nonsense, not because it's unlikely, but because if Dickens wrote soap operas, he wouldn't be Dickens. Dickens wrote lots of things, but we remember him for his novels. If he didn't write novels, he would have been doing something very different from what the entity we celebrate as Dickens did. What Dickens did was expand and exploit the possibilities of the novel. Change that, and you change everything. A truly great writer's greatness depends on an unlikely convergence of many different qualities, and the greatness usually comes from the writer finding the perfect form and style of expression for what is expressed.
Would Shakespeare be a screenwriter if he were alive today? Maybe. Probably. He liked making money and suing people, so I'm sure he'd find Hollywood more appealing than many other places. His scripts would have been chopped up, put through development hell, and rendered "accessible to a wide audience". Nobody but film buffs would know his name. And what he wrote probably really would have been closer to Romeo and Ethel, the Pirate's Daughter than anything else, because screenplays don't have much room for poetry. In other words, he wouldn't be Shakespeare-the-greatest-writer-in-English writing screenplays, he'd be some guy named Bill Shakespeare with a nice house in Beverly Hills.
Yes, Shakespeare and Dickens wrote popular art. So did Mary Johnston, Harold Bell Wright, Gertrude Atherton, Mika Waltari, and Henry Morton Robinson. Defenders of genre writing need to whine less and spend more time arguing the specific merits of whatever writing they are proclaiming to be masterpieces. Or they need to stop caring what the literary world they so dislike thinks of them, because in my experience, it's the genre-only readers who are more ignorant and disdainful of the world of mainstream fiction than vice versa.