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.
YSK, finding and installing mobi files are easy. Also, keeping your Kindle in airplane mode prevents ads. Fuck Amazon. Calibre is a great open source piece of software.
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.