for some reason (no real evidence behind this), but if I were going to bet on who would drop their e-reader line first I would bet Sony.
In conjunction with your examples of "giving up" on niche technology, I just think there is more at stake (money wise) for Amazon than Sony. Amazon is an electronic store front, that's their bread and butter and the Kindle is a mobile store front that caters to a pretty reliable buying niche (voracious readers
). The Kindle seems like a natural extension of their business model and a way for them to actually place a product into a less competitive market (as opposed to trying to sell a movie downloading box or MP3 player).
Sony has so many other options floating around that if the company took a large enough hit (or gave the books to the bean counters) they could easily decide that they gave it a go and the market is just not worth the time or effort.
In addition.. they market seems pretty handcuffed on innovation. screens come from a central location, and, while hardware can always be streamlined, most of the great leaps look to be software (and distribution) and Sony is not a software company. They make software for their devices, but you can tell in the design and length between updates (not only for Reader, but PSP, PS3, etc.) that they would rather just make the cool looking device.