The same could be said about iTunes/Apple - and musicians and music labels as well for audio books and ebooks Whether the owners of the channels become the new kings. There are plenty of choices out there of channel, but do the consumers want that much choice. Or like Google's dominance of search engines do they want an easy aggregated choice the works well in most circumstances.
Are writers really sharecroppers for publishers anyhow? Are they exchanging one master for another. (I'd argue vehemently that the key role of the publisher is not distribution or marketing but adding or creating value through the working with authors in the editorial and book-design process and then building a brand through what they publish so readers come to the publisher confident of what they are going to buy - and come back to buy again. Old-fashioned maybe.)