It seems to me that, having given some reasons for online publishing, it's only fair to give some reasons why traditional publishing should continue.
The first reason that springs to mind is the editing process. The editing of a work to make it suitable for publishing not only cleans up grammar, spelling and typos, but often makes a work more readable. Writing is often done for the writer - a thing many writers fail to understand. Editing is done for the reader.
Many writers can and do edit their own work, a valuable skill which, incidentally makes publishing cheaper and easier for the publisher. If you are a writer thinking of building up your skills, you could do worse than join Critters, an online writers workshop with a strong emphasis on 'critting' (offering critiques) of the works of other members. The discipline is good, and improves the eye you bring to your own work.
Secondly, traditional publishers are in a position to create a beautiful thing. A book as a work of art in itself, with beautiful graphics, a thing to treasure and look at with pleasure and even read sometimes.
Consider, too, those books for pre-schoolers and toddlers, even babies, that involve pop-ups and flip the flap, etc. People who don't know children may laugh, but books that involve some sort of activity are very important in developing a love of books and reading in children. Boys, particularly, need an activity rather than just a pretty picture in their book.
Thirdly, the mass market. Online publishing and print on demand is good for a small market, but when it comes to the mass market, scales of economy come into play, and publishers are in a position to take advantage of them. The sheer number of books being printed makes it possible for publishers to get a better deal on their printing. Then, of course, marketing and distribution comes into play, all of which publishers have been practicing for the last few hundred years.
This means that while online publishing is great for small niches and stories, poetry etc of an odd length, traditional publishing can take advantage of commercial opportunity. A book on a shelf in a book shop is a ready made gift. A customer has only to look it over and decide that this is the one they want.
Buying a book from a shop means buying an object, buying it for the look and feel, and even the smell of it. Downloading a book means choosing it for the text alone.