I think it's the last one, the two proven applications are book readers and price labels (and a few niche applications like readable displays for long-life battery-operated devices, Remarkable). There's not many potential users clamouring for a dumb terminal "laptop" (the battery life advantage would disappear real quick if you tried to compile large code projects) with E-Ink screen. Not that many people would buy a laptop that can't play YouTube videos or go on Facebook. Even I wouldn't want to write code on a laptop with the display latency of e-ink...