The point isn't whether backlist books make you rich, it's that if someone sells a used copy of an ebook you wrote it increases the chance of selling your backlist ebooks. It won't increase your chance to make money from your out of print paper books. And that's a reason that authors of ebooks should encourage the sale of used ones.
I doubt this is really going anywhere. Publishers have strong lobbies and they think in terms of paper books, not ebooks. I'd like to see sale of used ebooks become common but I don't really expect it to. I think it would benefit authors but not publishers and publishers serve their stockholders, not authors.
I probably should mention my qualifications for pontificating as I do about the publishing business. Unfortunately I don't have any. But that never stopped me from having opinions.
Barry