Ingram's is bigger than Microsoft and Walmart, comparitively. Microsoft and Walmart have competition, even though they are the most successful. (Though Microsoft may well be losing to Apple.) Ingram's has no real competition. You deal with Ingram's for re-orders or you don't deal, plus they do a fair amount of the initial orders. Nobody cares about Ingram's because they aren't visible, yet they control pretty much how books are sold, certainly how they are distributed. And if publishing moves into POD or e-books, they'll work to keep that control.
In the late 1980's, the assumption was that print books would move on to electronic books via CD-Rom and books would become like the music business. You could load on stuff about Chinese history with an Amy Tan novel, and lots of features with non-fiction. But the book publishers were barely into the electronic technology as it was and couldn't handle it, and the electronics companies lost any interest they had in a no-money field. Then the Web killed it off completely.
So then POD was going to be the big thing. It had a lot of potential usefulness for publishers, plus could get them out of the returns problem. But there are enormous problems with POD and the booksellers don't want it. Online sales are still only a relatively small slice of the sales pie. The book retailers don't want to be in charge of and dealing with the cost of production, don't trust customers on the demand part, don't want to deal with computer glitches, and need piles of shiny books physically there in their stores to draw in shoppers. So POD hasn't really developed.
E-books is looking like it's the return strategy. It is a good way to lure in tech-savvy book readers and to get non-book readers to buy books, or at least to get them to download documents and magazines. People like gadgets. But they've got to work out a lot of logistical problems first if it's going to work on a broad scale.
And here's the weird thing I've noticed: almost all new tech strategies that get bandied about for taking advantage of online and computer worlds seems to get shifted from commerce to publicity, when it comes to the book biz. Tor, for instance, is giving away free online PDR copies of novels as a publicity stunt that is working very well, though I admit I haven't gotten around to reading my batch. And similar stuff is going on with other publishers. They're using e-books to market, not to sell. The small e-book market, like the Kindle, serves as sort of a supplemental sales/pr campaign.
So I really don't know if or which technology is going to emerge for books' next step. It may be that all these technologies will simply increase the market for print books through publicity, which we can certainly use. But there's the paper crisis and the energy crisis looming on the horizon, right behind the global warming/water shortage crises. So Amazon is covering all the bases, I guess.
This is not going to stop people from self-publishing, and may create more outlets for them to do so. Small presses and e-publishers who rely on POD may fail, but then they have a high turn-over rate anyway. The smart and lucky ones will continue. And Amazon probably won't stay isolationist for long.