Well, you can certainly buy a large-screen eInk device if you want one
. I just think it's unlikely you'll see one from Amazon. They're concentrating on the tablet market when it comes to large-screen devices.
I strongly suspect that within the next 10 years a screen technology will emerge (perhaps LiquaVista, perhaps something else) that will give daylight-readable good quality colour screens with low power consumption, and that once that happens the distinction between tablets and eInk readers will disappear, but I think we're a few years away from that point yet.