At least for ebooks it recently occurred to me that it will probably be difficult for any ebook vendor to establish a monopoly. After all, everybody can just offer ebooks on their web site.
Hatchette and any other publisher could stop selling their books to Amazon tomorrow. You could still buy them in paper from loads of physical stores, and you could still buy their ebooks from dozens of places, including little-known and difficult-to-find resellers like Apple and Google. But if they lost, say, 40% of sales, would we call Amazon a monopoly?
I don't know. My inclination is to be pretty conservative in applying heavy-handed regulatory remedies. That's a big hammer, and it's one I'd rather not use on a whim, and right now, I think it's still far too easy to compete with Amazon on its core business for this to be an option I'm ready for. But I'm not sure how hard it would be to convince me otherwise in the future.