A4 e-ink has considerable audience that doesn't know they need it – people who read papers. All the academia is formatted for A4 and stored in PDFs, which is pretty unreadable even on 10". People literally print them on paper to read.
I currently enjoying A4-sized Boox Tab X and it's a game changer, as I read a lot of papers and want to read more outside. Love it so much, and having Android is important (instead of some own walled OS), as I can sync my papers via ReadCube Papers app. I also tried to use it as a monitor for coding, but even with their superrefresh technology refresh rate is still a problem.