This is as I understand it. I've not read the Kindle agreement in detail.
If Amazon discontinues the product, you have no recourse for the books you've bought. When kindle fails, you lose the books. Amazon needs to guarantee to offer people a cheap hard copy path or other ebook path if Amazon bails on Kindle.
This has happened with pro baseball and your ability to download games. They canceled the old service and hosed everyone who "owned" old games. It's happened with any number of music services. And it will happen with Kindle at some point, even if through obsolescence. What happens to your books then?
This is the pitfall of any Digital Rights Management product.