Back in the old days I had a Hercules monitor with slow phosphor. I've tried to find some figures on the response rate, but I wouldn't be surprised if it was a similar speed to the eink screen. It was really comfortable on the eyes and I slightly miss it (of course, it was the size of a postcard compared to a modern monitor). So I think an eink monitor could be quite usable for certain purposes.
The thing I'd be concerned about is scrolling. We've got very used to monitors which will scroll smoothly. I've seen monitors which just turned into hash when scrolling; there's a reason why vi and emacs have modes for page-at-a-time scrolling.
https://www.visionect.com/development_kits
Greyscale: $3600. Colour: $4700. There's a 13" model at a mere $999, but who wants that?
Slightly more realistically, there's the Dasung Paperlike:
When smooth scrolling, you lose a lot of time following the animation and waiting for motion to stop.
For some use cases, this would be ideal. For others, not so much.
Can you speedily grayscale into black-and-white? That is, type a letter, putting black/gray into a previously white space (or vice versa, if using a dark color scheme), clearing a single-pixel-wide cursor and installing a new one to the right of the added letter. I don't need the letter to be fully black instantly, I just need to see that I have typed something.
Sure it won't be as fluid as 1080p @ 120Hz, but I'd like it anyway.