I don't think monthly rentals is the way of the future. Some months you feel like reading; others, not so much. Also, €5/book seems a tad overpriced when you are able to buy lots and lots of e-books at $3 and less.
There is a phenomena at play in the e-book business that doesn't get discussed very often. Early adapters (and that is basically us) are hard core readers. Most of us has read ferociously since middle school, we were the geeks (more often called weirdos) back then. Now we are entering a stage of more general acceptance, the occasional readers are getting in to e-reading. And some of them will move towards hard core readers, most will remain occasional readers. It doesn't take them long to find out about Gutenberg and other sources of free classical literature. Works they didn't get into when the weirdos could quote Brontė sisters and Jules Verne. So suddenly they have this whole world of literary classics available free of charge. They will happily explore those options rather than a subscription service.
Then again, I could be left out in right field all by myself.