Somebody smarter (or more patient about wading through data) than I am could probably figure out how far along this bifurcation is already, but Amazon is doing its very best to build a body of content that is desirable and available from nobody else but them.
This is something you can do when you’re in the neighborhood of 70 percent of ebook sales and already more than half the total sales for many works of fiction, which is where the self-publishing world is strongest. It is not an opportunity that is really available to any other retailer. Apple has given it a try for more complex ebooks for which they provide ebook-building tools and, presumably, offer the most productive distribution environment for complex content. But they’re playing on much less fertile ground and they don’t have anything like the audience share necessary to drive this strategy very far.
It is hard, if not impossible, to imagine that any other ebook ecosystem could offer benefits that would make it worth skipping Amazon.