This really shows the problem with this model. Textbook publishers have been very successful at attracting the top authorities in their respective fields to write the textbook. Sure, anyone can rehash what's already written in any other textbook. But can Saylor get someone like Halmos or Axler to write their linear algebra textbook?
Just because you're the top authority in the field it doesn't mean you're good at teaching it. Writing effective textbooks is a skill in it's own right.
