This is based on my key theory. (Technically, there are spoilers. But it's spoiling the first chapter of any given book, so...)
I've been reading the Vorkosigan series recently. It's an interesting series, and because I'm primed on the idea of tying the audience to key characters (or things), I'm coming at it from a slightly different angle than most people would, I think.
The Vorkosigan series is a clear demonstration of how emotional keys work - and fail to work.
The books focus almost entirely on the central character: Miles. He's obviously the emotional focus, and in the first book, he is clearly the key. Every other character exists solely to make us feel more for Miles. He does audacious things, has fun friends, beats challenges with his great big brain, and in general raises hell.
By the end of the first book, we're very attached to Miles.
The next stories are interesting and well-written, but they aren't about the same character, at all. Sure, it's Miles. But it's Miles Vorkosigan, not Miles Naismith, and that's just not good enough. It adds a lot of depth to Naismith to know that he's actually a frightened boy out of his depth underneath - but we're not attached to that frightened boy. We're just using that boy as a hook to latch onto Naismith even more.
Vorkosigan is not a key. He is just a hook, allowing us to attach to a key.
In my opinion, the author makes even more dire mistakes later.
Like Vorkosigan, the Dendarii fleet is not something we've invested in for itself. It only exists in relation to Naismith, the same way Vorkosigan does. The fleet is a hook which lets us attach more "gee, isn't he the greatest" vibe to Naismith.
The fleet is not something we consider an independent entity, any more than we consider Vorkosigan an independent entity. Watching it do things without Naismith is kind of like watching someone's right arm wander off and chat up the nearest girl. It's kind of interesting, but it's both bizarre and somewhat repellent. It turns into a kind of bleak comedy when you really like the person who is chasing after their own arm.
Lois McMaster Bujold (the author) is a master (mistress is more correct, but definitely gives the wrong impression) of focusing the reader's emotions into one central sink. What she doesn't seem to realize is a basic fact: a reader is invested in the thing she wrote. It's not the personality of the character, or even the character himself. It's the character's situation, everything about him, from his dialog to his assistants to his fleet.
Every bit you take away reduces the strength of the connection between the audience and the key. Every hook you used to snatch emotion into him, when removed, loses that much emotion.
Lots of people love this series, and it is very well written. But it takes until Mirror Dance (which is, I believe, the fifth book) for Bujold to develop even any secondary keys. Vorkosigan is the only key in the first four books. He's the only person we feel for. He is our Captain Jack Sparrow, our The Force, our Robin Hood. All of the things that happen are hooked directly to him, pulling us to him.
In Mirror Dance, they drop him to a background character. The first half of the book is spent in screaming agony as hooks are pulled off, scattered around, wasted. Reading it was like pulling teeth. I simply do not care about the other characters. In fact, several of them I have grown to strongly dislike, because they were made into heels to make me like Miles more in earlier books.
The last half of the book is interesting, but it isn't at all a Naismith book. And it ends with Miles diminished.
I don't think that was the right move, but it was fascinating to watch to Bujold's artistry slowly shift gears. In Mirror Dance, she played with using multiple keys - Mark, Thorne, even certain clones were made into more than just hooks. They grew into keys of their own right. Minor ones, but that's fine.
So, what am I going on about, with an arrogant review of a popular book series?
Actually, I'm leading up to diversity.
You can't keep spiralling in the same orbit forever. One person (one key) can only be plumbed for so long before the well runs dry. Miles is an unusually (unnecessarily?) complex character, but five books (four books and a collection of short stories?) is still a very long time, and most of the books are backtracking. I can only imagine that the remaining books will use other keys to try to keep my interest, because Miles is plumbed.
Now, in the process, Miles became an icon - an unforgettable part of Life, the Universe, and Everything.
But then it's over. There's no more Darth Vader: you explored him, he died, end of story. Well, except for the backtracking, which might have been cool if it was written by someone who knew how to write.
That's the point: icons cannot go on to bigger and better things because they have hit the extent of their power. If they get any bigger, they stop being who they were and start being someone else. That's not necessarily bad, but it does split and mush up our key enough that it stops being an icon.
If Miles continued on to conquer the known universe, it would probably still be a fun read. But it wouldn't be about Miles, any more than Mirror Dance was about Miles. It will be about someone Miles has become.
You can avoid this problem by using more than one key.
If you develop multiple unique individuals and get the audience invested in all of them, you can use them in combination and permutation. You have thousands of times more stories with a mere three or four keys rather than solely one.
Perhaps that's what Bujold finally decided, when she sat down to write Mirror Dance. She almost certainly thought Miles was running out of rope - because he was. He had already turned into an icon by the end of the first book, and it was concrete by the end of the third. The only way to continue the series was to put in some other characters we felt emotion about, and in order to do that, Miles had to be shattered.
But you shouldn't have to shatter your mythos to save your mythos: from the very beginning, build in multiple keys. It is a bit harder to manage, but it's sustainable.