I would argue that e-readers actually do need reasonably low latencies. The reason I still read books is because I can flip through pages quickly and glance over 20 pages a second as I look for the page with a certain pattern of text or an image.

IMO eInk readers are only good for "linear fiction", books that are read from start to finish.

Anything that needs to be browsed or flipped through, nope. Very few electronic versions are usable in this case, be they normal LCD screens or eInk.

but you don’t need that for signage, calendar on your door, paper decorations on your bedroom walls, meeting room reservation sheets...

Are those use cases significant enough to make e-ink "take the world by storm"?

Not sure how specific you're trying to be - here's a specific example of 4.2" at $35. I actually would be interested to see a graph of cost-per-square-inch to see where the arms and legs actually do come into play.

https://www.aliexpress.com/item/400x300-4-2inch-E-Ink-displa...

https://news.ycombinator.com/item?id=15721766