Reduced eye strain and battery life are the two big advantages of e-ink, neither are an issue for someone reading 20 minutes a day. E-ink readers really only appeal to avid readers which is less than 20% of the population, and even some of those people prefer reading on backlit screens. I love my Kindle, but would never recommend it for someone who reads a few books a year, they are much better off with a multipurpose tablet or a paperback they buy at the airport once in a while.
I'm not sure why any of this is a surprise to people, e-ink readers are a niche and will eventually reach saturation and the sales growth will stop. I don't think that means e-ink will disappear, it's still the best technology for that niche and someone will continue to make replacements, probably Amazon in order to maintain their dominance in ebooks.