This is not an anti-Kindle rant. I have purchased (rented?) several Kindle titles myself.
However, YSK that you are only licensing access to the book from Amazon, you don’t own it like a physical book.
There have been cases where Amazon deletes a title from all devices. (Ironically, one version of “1984” was one such title).
https://www.nytimes.com/2009/07/18/technology/companies/18amazon.html
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.
Here are the terms of use:
https://www.amazon.com/gp/help/customer/display.html?nodeId=201014950
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’m glad you mentioned airplane mode. I noticed something interesting about my Kindle after I set up a Pi Hole on my home network. The kindle would constantly try to connect to the Amazon mothership. Because the Pi Hole was blocking it, it would try it over and over again and this quickly depleted the batteries (maybe trying to boost it’s WiFi signal? I’m not sure). Putting it into Airplane mode helps preserve battery life noticeably, back to what it was before I installed the Pi Hole.
It drains battery faster even if it’s not blocked too, kind of annoying.