I bought an early e-ink reader (Sony 500) then moved to a Cybook because Mobipocket content was a easier to get. Bought another for my wife, then she bought a Sony 505, again for the content, then I put the kindle app on my iPhone (content again) and now I have it on my iPad etc... etc...
The older models aren't worth selling as they're not worth much, and they still have the occasional use e.g. holiday reading in bright sunlight. So yes, it's just a general upgrade process, without the disposal
Pete