If you're talking about certain types of fiction and non-fiction (e.g. novels, history, popular science), then I agree with you. However, there is more to the world than that. Some books need more sophisticated layout and incorporate more complex elements within the text (e.g. graphs, equations, tables, etc.). In the cases, PDF is more suitable as an ebook format.

Please remember: just because epub, mobi, and other reflowable formats work better for the types of books that you read doesn't mean that it works well for all of the texts that other people read. Personally, I prefer a mix. The reflowable formats are best for the novels and certain types of non-fiction works that I read. PDF is better for the technical books that I read.

It is not an either-or situation. The world is too complex for that.