There are a lot of books out there to use as training data. Fifty years is plenty of time to perfect the plagiaristic mashup. I expect autowriters to dominate the market for "boy meets girl, boy loses girl, boy regains girl." stories that are set in the past. There might be a job, half novelist, half computer programmer, done by humans who tweak autowriters to add up-to-the-minute details to their stories.

I doubt that "I'll even settle for a book that can rival human authors." states a sharp edged criterion that separates before from after. Crossing the gap between rivaling Barbara Cartland and rivaling Tolstoy mind take a century of software development.