Amazon got some bad press when they "pulled back" a book that had had copyright issues. There was enough of a stink that they have made a very clear statement that it won't happen again; if you buy a book they assure folks that it will stay in your device & account library until you remove it.
Amazon book purchases remain available for re-downloading. If you delecte it from the device it will show up in your "archive" listing, from which you can re-download. Also, you can have mutiple kindles associated with an account (6?). Any device on the account can access any book in the account library. The Amazon DRM does limit use of books to devices associated with the purchasing account.
Baen Books, on their webscription.net site, retains info on your purchases as your account history. At any point in time you can re-download books you have purchased there. As their books are DRM-free, you can download in different formats to read on different devices without having to associate them with an account. They also have a large free library where you can download full version books from current authors for free as samples of their work, betting you will come back & buy books from authors you like.
From what I've seen on the forums, the keyboard wear issue on the kindle was mostly associated with units with "clay" style keys, most have a different style of plastic. People have been getting replacement kindles issued when the problem manifests.
Nick