That was based on the assumption that a capable multi-purpose tablet could only have a few hours of battery life. The iPad’s ～10 hour endurance was a huge game changer.
Comparably-sized e-ink devices offer days to weeks of battery life.
Most especially when used as an e-book reader (e.g., occaisionally invoking a full-screen refresh, without illumination, radios, or other processing), but even when used as a full-on tablet easily exceed a day of typical use.
Of course, but 10 hours is enough to be very competitive with readers for most use cases. It's enough that if you go away for a weekend and forget your charge cable, and your main use is reading ebooks, most people would be just fine. It makes it much more competitive than anyone could assume before it came out.
My experience is that battery life tends to degrade over time. 10 hours is the best the device will ever have.
e-ink offers days to weeks (with judicious use). Even with degredation, 1-2 days' usage without charging will be an option. What I've found is that charging is quite rapid, so even if short top-offs are all that are possible, those restore a large portion of total battery life.
And the device is readable in bright light / direct sunlight, again, a major benefit.
