I disagree. I think digital books will broaden midlist authors' readership. The idea that most people discover new authors by browsing a bookstore is outdated. Most readers discover new authors through word-of-mouth recommendations, social networking, online browsing/blogs, and book discussion groups. Yes, sometimes recommendations come from bookstore clerks, but it's only a tiny part of the overall picture.
Additionally, midlist authors often have independent publishers that can and do price e-books so that they'll sell in volume. There's never been a better time to be a midlist author
L.J.