I'd love to have a dedicated ebook reader with a Liquavista screen. I have absolutely no need for video, but I do have a need to view things quickly, which present electrophoretic screens like E-Ink cannot do. There are many potential benefits of a high-rate-capable display that do not amount to video playback.
E-Ink's current manifestation probably deserves to be obsolesced with things that perform more quickly and with better viewing quality. However, one should not assume E-Ink to be equal to dedicated devices, unless E-Ink were significantly cheaper than competing e-paper technologies, which I don't expect to be the case.
For competent video playback, there's more than simply a screen to be concerned with, and cumulative hardware and software costs could be a factor in device price. If such is the case, then dedicated readers with superior screens should still be able to replace E-Ink devices assuming prices are kept similar (or lower).