This argument is akin to saying "now that anyone can publish a blog or tweet, what's the point of news places like NYT or WSJ?" I don't believe either negates the needs or benefits of the other. In addition to what others have mentioned, with publishers (just like with established news organizations), you can (usually) expect a certain level of standards and thoroughness with the end product, at the cost of fewer and less diverse works. With self-publishing, you can find end products that might have been passed over by publishers for various reasons (not a big enough market, too controversial, etc). Each is beneficial, each is necessary.