It doesn't matter if you've had a Kindle since the first generation or if you've just purchased your first one. Amazon can delete books that you've purchased, and you'll no longer have access to them. While this is rare, it still happens.
In 2009, copies of George Orwell's novel 1984 were deleted from users' Kindles because of rights issues, and it could happen again. Even worse, account bans from returning books can render your digital library useless.