I think that's likely to be true for a couple of reasons. One is the fact that, as Nathan points out, publishers know how to produce a decent-looking product. Editing, copy editing, and proofing are hard work. Cover design still matters, even in digital publishing.
But aside from that, look at blogs. A lot of blogging services are free, and all are by definition, web-based, so in theory, anyone with the talent and drive to attract an accidence ought to be able to do so, right? Maybe. And maybe not. Plenty of folks have become famous from blogging, but how many have done so totally on their own, just from the insight and quality of their posts, with no name recognition? Many really well known blogs are associated with the business about which they are blogging. Nathan's own blog is a case in point. The reason so many people follow it is because he was already an established agent when he started writing it. Being an agent made him good at blogging about it, not the other way around. I'm not saying it's impossible, only that it's rare.
Just as there are a gazillion blogs, there are a gazillion aspiring writers. Being able to put your book out there for sale via the web doesn't mean that the world will automatically beat a path to your site in order to buy your books. The move to digital publishing will make it possible to truly self-publish, but it won't make it easy. A digital book still needs to be promoted to its audience, once way or another. Stephen King might not need a publisher but the rest of us could use some help.