I don’t think they still be niche if they were competitive with LCDs in other aspects, most importantly color gamut and ability to show video (I think longevity and power usage already are competitive, if not more than so).
So, why? Are they inherently more difficult or expensive to manufacture, do they use materials that are too rare?
The way e-ink works is quite different from an LCD. They didn't even get color until recently.
Personally, I think the benefits of reflective reading surfaces (ie e-ink) over backlit reading surfaces (everything else) is massively overblown. I have an e-ink tablet now that they are >300ppi and it's nice to read on, but it's a novelty. Some people love 'em, but that hasn't stopped the iPad Pro from outselling all of the e-ink tablets by a huge factor. I think e-ink's future is mainly in small/low power/ambient displays, like price tags or luggage tags.
Most people read most things off of their phones or tablets and it's just fine.
What about a hybrid display for portable devices. A transparent OLED panel sitting over an e-ink display. You could have the battery life and readability of e-ink and just turn on the front panel when you needed full color or fast refresh rates such as for video.
If I could bind a 8/10" eInk tablet that was around $200 (or $300) on which I could read and reply to my emails, read RSS feeds, check my calendar and take handwritten notes on it (that I could export/sync) I'd constantly use it.
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So, why? Are they inherently more difficult or expensive to manufacture, do they use materials that are too rare?