I don't disagree -- remember, I'm one of those author/editor combinations myself -- and I grew into a publisher. David Pogue is following the same path right now. An author becomes successful, and they try to extend their leverage. Not all do, but the possibility is there. Tom Clancy has a game publishing company, a film production company, and so on.
Not every artist wants to go this route, but many do. But in a rich media ecology, you have the opportunity to choose which pieces of the problem you want to own. David Pogue wants to own his editorial and production, but doesn't want to go all the way to building his own sales force, so he works with O'Reilly as a packager. In my early years, I made deals with other publishers to distribute my books internationally. Now I do it myself.
The self-published author you describe above *is* a publisher. There have always been lots of one-book publishers. The new medium hasn't changed that. Nor has it changed the fact that, having had one successful book, a self-publisher seeks another, and another, and another...and after a while, doesn't write them all him or herself.
The web is a perfect petri dish for watching all this in action. Yahoo! was once two guys at Stanford, not all that different from a whole bunch of other early websites. Now they are a major hub in the web content distribution chain. Whether you call them a publisher or a distributor is somewhat irrelevant. The point is that distribution hierarchies don't remain flat. Once they get large, nodes of specialization emerge. One of those nodes we call a publisher.