I suspect that book publishers introducing their own digital distribution mechanism would not make the traditional book distributors (e.g. Ingram) very happy, and you don't want a major channel partner getting angry with you.
Because Amazon merely is a competing distributor, rather than a publisher/vendor trying to go vertical, distributors have less leverage.
That's a weird way to ask it. iTunes has competitors, after all. Just today I bought a slew of mp3s from the Amazon store, because they offer incredible deals.
If you mean will Amazon become the majority store, the answer is that they will be unless something better comes up. Isn't that how it always is?
Because Amazon merely is a competing distributor, rather than a publisher/vendor trying to go vertical, distributors have less leverage.