You're missing crucial data here
«I'm fairly willing to bet that once your two-year warranty has expired, so do your books - if your device goes kaput.»
I don't know about B&N, but Kindle books can be stored on your computer, your Kindle (if you have one), and Amazon's cloud. Did you accidentally delete one of your Kindle books? Pull it back down again, no charge. Same deal if you have a Kindle & it gets stolen or broken.
Apple's iBooks are at least on your (iTunes-based) computer as well as your iPad/iPhone. I'm not sure what their policy (or B&N's) is about re-downloading, but I wouldn't be surprised to find it similar to Amazon's.
Dead-tree books have advantages, to be sure — I always buy dead-tree versions of field guides for their color illustrations, for example — but you're glossing over some of their disadvantages: once you get past drying them out or taping torn pages, the only answer is to buy a new one. They can't be replaced for free if stolen/destroyed, they can't be updated. Then there's that whole storage thing… if you have 1500 ebooks and 1500 dead-tree books, and you have to suddenly evacuate your premises, you can grab one smallish gadget and have your entire library with you.
The best way to ensure longevity of ebooks is to buy DRM-free books and convert them to newer formats as needed. It's harder to kill-switch a non-DRM ebook too — 'cause it isn't necessarily on the device in question.