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.
Yea Audible too. I can't remember the name of the tool but you can connect to your account and it pulls all your purchases locally DRM free. It was handy for setting up Audiobookshelf
Just tried out Libation for the first time this week, very happy so far. Further testing of results is still required, but this was an excellent suggestion.
I'm pretty sure it's less that you can crack the DRM on the newer format and more that you can get amazon to send you a version that's compatible with older devices (which uses the older DRM).