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.
I am now of the opinion that you should just download books off indexing sites/IRC/ Usenet/torrents and if you like the book and want to support the author, buy a physical copy, or buy 2 and put one in a neighborhood free library. That maximizes the good you are doing and helps your community instead of just generating Bezos bux.
Some authors straight up tell you where they get the most money for your purchase. Hardcopy is almost never it. But also those mini share libraries are cool and I like dropping sci fi books in.