Tim brings together a lot of my thinking here and lays it out very clearly!
Without arguing the point, I see a 20% market as one in which bookstores continue to survive in significant numbers, publishers continue to have an important role in the book world, and public libraries mostly carry on as usual. At some higher level I see most bookstores vanish, non-academic publishers largely disintermediated away, and public libraries in crisis. (For libraries see my posts on ebooks in libraries.)
Feedback loops. My contention is that ebooks rise far above 20%, becoming the dominant book format, because the logic of ebook success has built-in feedback loops. You can call it “success breeds success” or “vicious cycles” as you like. Same idea.