I suspect that most people who want a color display want something with a fairly high refresh rate, such that you can play (for example) movies and games on it. I could be behind the times, but from what I recall about the technology, trying to get an e-ink display to update at that speed would be very difficult, battery-draining, and generally unsatisfactory. So I would guess the back-lit color devices are likely to stick around indefinitely, because the color e-book market is only an incidental one for them—when color e-book readers with e-ink are available (especially textbook-format), the iPads of the world will lose out on that market, but they'll still be used for the sub-netbook computing-device market that they're really aimed at.