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 have a few kindles, have upgraded over the years and have been able to use them all in the same manner:
With a new device I connect it to the internet and update the firmware to the latest version (the factory installed version has had a lot of missing functionality in my experience). Then I block it from my network, delete the AP entry and put it permanently into airplane mode.
When purchasing an ebook from Amazon you can download it for usb transfer and I organize it on my laptop with Calibre.
Calibre can also strip drm, but if you’re transferring it to the device you downloaded it for it isn’t necessary.
Amazon may at some point in the future change all of this, but the content I have already downloaded can not be revoked and is usable outside the Amazon ecosystem if the drm is removed.