As for Kindles only lasting a few years- they may introduce a new version, but the beauty of computers as appliances is they will last as long as their battery and 120 volts hang around. I have an ipod from the beginning of ipods; the only reason I haven't repaired the battery is I no longer depend on it to listen to music.
Lastly, before you worry if your Kindle will last for the decades-to-centuries range, and elect to use books instead, recall that books themselves must be carefully printed, stored and used to last 50 years.
It's a good job that some of the stuff before Windows95 actually got printed out.
ps. suppose Amazon decide to stop Kindle, or get a better deal from the publisher - or just decide all your existing ebooks don't work on kindle 4. Just like Walmart did with their online music service. With the wonders of DRM you can't back up a copy of the ebook, it only works as long as Amazon's licence servers work.
Should Amazon decide to exit the hardware space, there would still be the kindle software for other platforms.
The difference between Amazon and Walmart is that Walmart's music service was far from their core business. Amazon's business is to sell things, especially books, online. That's why they don't limit their ebooks to the kindle device. They don't care if you're using an iPad or a Blackberry, so long as you're buying some of your books from Amazon.
Walmart's core business is to sell things in stores. I'm not sure what value an online music store really gives them, so I'm not surprised they killed it.
And that's for files that I created on hardware I own.
Amazon as a company is only 15 years old - will it be around in 50 years? Any given Kindle will only last for a few years - will I always be able to afford to buy a new one? Will they always be available to buy?
It is a big issue for me that I can't buy an ebook and have a reasonable expectation that the ebook will be mine for the rest of my life, and can be given to my kids to read.
It makes me think of ebooks as disposable - I can't build up a library. It's more like borrowing someone else's books where they can demand the book back.
But, like this guy, I still use them for convenience...