They typically can do localized refresh now; so you start with a white background and can make text function like it does normally when typing.If e-ink could be used without refresh lag, it would literally take over the B&W flatscreen market. But after all these years, the tech hasn't really taken off due to that problem. They've reduced the time required for refreshes down to about 3 Hz, but compare that to 60Hz and higher for normal LCD screens, and you can see why e-ink has serious usability issues for anything outside a basic eBook reader.
Video is really the only thing you can't make useable now. For what the Macintosh Plus was capable of; I think it could work. Refresh rate is still gonna be faster than most of the processing.