There have also been cases where a customer violated Amazon's terms of service and lost access to all of their Kindle e-books. Amazon has all the power in this relationship. They can and do change the rules on us lowly peasants from time to time.
Note, there are indeed ways to download your books and import them into something like Calibre (and remove the DRM from the books). If you do some web searches (and/or search YouTube) you can probably figure it out.
If someone want an alternative there is ebooks.com. The two titles I got from there were way cheeper than on the store linked to my device. No nonsense app or anything, just download the file from the store and do what you want with it.