They are dirt cheap but only a handful of chinese manufacturers are using them.
On top of the e-ink IP stronghold, I think there is also a Product manager issue in consumer electronics. Everyone wants to show case Macbook-esque glossy displays whereas a large number of consumers want displays that they can use outdoors and without eye strain. Everybody complements garmins transflective displays but somehow nobody makes the leap of slapping them on phones or laptops.
Not only could you then use any device on the market, but your books are truly yours. Immune to Amazon revising them in the future, and immune to having your books revoked. They'll last as long as your preferred backup methods last.
And that's a good reason to do this even if you keep using Kindles.