E-ink is not practical either. The power needs to be boosted to ~100v (power hungry), each key would need a separate screen, and would have to be manufactured to be tiny. E-ink alone is expensive, probably even more so than OLED, much less color e-ink! Which is still in prototypes for many companies. Color e-ink will not hit mainstream consumers for a good 5 years.
Either way, you are looking at huge manufacturing costs, poor durability, and sucky key quality.
Oh, and future kindle? Uses a light buide printed on top of the screen. To do this with keys, the LEDs will have to be on top of the keys, which again makes backlighting very difficult.