I don't worry too much about longevity of devices. I mean, if I haven't already replaced it I will someday drop my Sony reader and damage the screen. Isn't that inevitable? Or do you really expect your rechargeable battery to be purring along just fine after five-plus years? I don't. These are not yet "forever" devices. Maybe next year.
Along those lines, I make a point of avoiding device-specific purchases such as books in DRM'd LRF. Never know when I'll want or need to read on something else.