For indoor use it has to achieve a lot more. But I would use one as a primary display to rest my eyes more.
Greyscale works as syntax highlighting, you do can get used to it after time. But what greyscale does to you is that isn't that interesting to you anymore to look at the screen, so you do start get "bored" from it and the eye starts to wander around looking for something else perhaps pretty to look at. Nice if you want to look often enough out of the window, but annoying if you have work to do.
For at dark at night coding it isn't that bad. But having an AMOLED with the right colourscheme and low lightness setting is more pleasing I found.
An e-ink is enough to program on emacs all day, but color highlights is a missing feature.
Maybe when patents expire, the tech will improve faster allowing for acessible color ones.
From my experience using an Kindle it is way less eye strain, and reducing the amount of strain, in this case hours every day, would have to have a positive effect over time.
Maybe in the future it will be normal to have dedicated E Ink programming displays.