When Stephen King emerged as a best selling author in the 1970's and 1980's, there was a boom in the publishing industry. Just about any novel with commercial value had a shot at being on the best seller list. This was also a turning point in the industry where novels went from being a literary pursuit to being a large outlet for commercial fiction. Stephen King was an example of an author who was in the right place at the right time, in addition to being a very good writer.
Along with this boom in commercial fiction came all these imprints and divisions in each genre at the big New York publishers. This included horror, the genre Stephen King excelled in and helped make famous. He knew the subject matter of horror very well, reading the works of H.P. Lovecraft since he was a child. Yet also knew how to make it his novels outside the genre and appeal to the mainstream, inspired by the realistic settings in the works of Richard Matheson, someone he read since adolescence. He followed a similar and became a bestseller.
The point of view in Stephen King's book were in third person omniscient. A godlike point of view exploring the viewpoints of different characters in the novel. He either a whole canvas of characters to work from, or a dual battle between two main characters in each novel. Yet such things would not work in today's publishing market. He would now have to write his novels in third person limited, a more investigative or explorative point of view from the eyes of the main character. A very different approach from the one Stephen King learned in his prime over twenty five years earlier.
Also, horror novels are no longer in demand. While horror movies remain a big draw at the box office, horror novels have dropped off the radar to the point where no large publishers in New York have any horror imprints. Outside of Stephen King, horror novels have been relegated to bargain bin and online releases by small presses. Both he and Dean Koontz are among the few horror authors who remain on the fiction best seller list. Everyone else at the top now is writing a fantasy series or suspense thrillers, the latter genre also involves Koontz.
The era of fiction where just about everyone can be a best seller is also long gone. That sort of market dried up by 1990, and publishers took a more career oriented approach with their authors. In the 1990's, many authors were often limited to one genre with three books in that genre to prove they were a success, and the next two to cement their careers before any attempts at another genre. This more recent method of focusing on building careers has also changed, and most authors in the 2000's were lucky to even have one full length novel out in bookstores.
The truth is,
many of Stephen King's horror classics would not work in today's market. There is no longer is a place for horror in commercial fiction. Such novels these days would most likely end up in a bargain bin after being released by small press publishers. If released today, King's most famous novels would more than likely be classified as suspense thrillers or fantasy to succeed on The New York Times Bestseller List.
The low key suspense thrillers he wrote under the pen name Richard Bachman would have found a huge audience. King, if he were a recent bestseller to debut in the last decade, would be competing with the more recent bestsellers releasing suspense thrillers. Along with the more established suspense thriller author such as Tom Clancy and John Grisham. Yet the public would know him as Richard Bachman at this point, and not as Stephen King.
His Dark Tower series still would have sold very well, since fantasy has became a hot selling genre in the last decade. The Dark Towers would have blown Stephanie Meyer and her Twilight series out of the water. While also serving as stiff competition for J.K. Rowling and her Harry Potter novels. Who would not pass up the chance to see a head to head battle on the bestseller list between the feud between Roland Deschain and Randall Flagg or the confrontation between Harry Potter and Lord Voldemort?
Either way, Stephen King still could have been successful as a bestseller if he was a more recent novelist. There is no doubt he would have had to struggle a bit more and limit himself to fewer genres, much less limit himself to one genre. This would also mean limiting himself to releasing fewer novels, since the publishing industry has seen a sharp decline in the last decade or so. Along with fact this is a more commercial marketplace than the literary one Stephen King helped revolutionize and innovate. In this era, he would not be the game changer he was, he would just be another bestseller.
However, this does not mean he would not have written horror, it just would not have been as successful. The Richard Bachman and Dark Tower novels would have made him successful on the bestseller list. Horror novels in this era would have been more or less passion novels that he wants to do. In his prime, horror novels were what sold while novels like those under the pen name Richard Bachman and The Dark Tower series were those he wanted to do. In this era, it would have been the other way around.