You do this by curtailing the capability, but leave enough that it's still tremendously useful. Eschew a backlight and do everything with the e-ink display. (They'll have to get more responsive for this to work.) Keep things on a low bandwidth basis, possibly even turning the radio off most of the time.

Something with just a little more capability than a "dumb" phone or feature-phone but with a more comfortable interface and 7 days battery life would be able to capture the market. Let's say, one builds "visual voicemail" and add some sort of more user-friendly banking front end which still uses the current SMS channels. Combine that with light web browsing and something like Wikipedia and Twitter/Facebook, and you'd have it.