Sometimes it's the author, not the story

I have made reference to an epic fantasy series, The Wheel of Time, several times before on this blog. The series’ author, Robert Jordan, died in 2007 and left the story incomplete. Jordan had made it to book 11 over the past 20 years, but the finale was left unwritten. So I hear, as I stopped keeping up with the series around volume 5 (I got bored with it and never finished). Nevertheless, The Wheel of Time has interested me because it was an empirical test of issues which crop up in literature. For example, it was noticeable when I was reading the series that Jordan’s plotting was becoming torpid, he was losing control of the various threads holding the narrative together. From what others have told me this problem continued and waxed as he cranked out book after book. So I wondered, is it simply not possible for a story to span so many pages? Or, was the author running out of juice?
Unfortunately for Robert Jordan, his death set up a perfect natural experiment. The fantasy writer Brandon Sanderson was chosen by Jordan’s wife to complete the series. So book 12 just came out, The Gathering Storm. Below are the average scores from Amazon for all of the books in The Wheel of Time.

It seems from this that Sanderson’s contribution was well received. He took Jordan’s notes to construct the narrative, so clearly it was the execution which revived reader enthusiasm, not substantive flourish. The problem with The Wheel of Time wasn’t endogenous to the nature of the story then, rather, Jordan had a hard time keeping it all together and maintaining enthusiasm across so many books. The Wheel of Time made him a rich man, but one has to wonder how much interest he had in the destiny of Rand al’Thor after 25 years (he began writing The Wheel of Time in 1984). Harlan Ellison stated once that he avoided sequels because if he couldn’t say what he had to say in one book it wasn’t something he wanted to touch.
Note: One minor issue which I think needs to be considered: for many authors pulled out of a slush pile you see “regression to the mean.” To break into the business they need to produce something that stands out, and so naturally the first book is more likely to be an exceptional production by the writer, with later works reverting back to their natural level of quality.