Re:When will they become mainstream? (Score 1)

Your criteria are already there... sort of...

Ebooks are no more expensive than paperbacks.

With a little effort (after, sadly, quite a bit of searching for format-conversion apps), an ebook is easily converted into mp3 files. (I use the excellent TextAloud, but there may be others...)

Then you can listen to your book anywhere, even more easily than a paperback: in your car, while jogging, while having that lonesome dinner on the road, ...