That’s a real concern. Fortunately, it’s easy to make a backup of your Kindle books. Just connect your Kindle to your PC via a USB cable. At that point, your Kindle will appear as another drive on your PC. Now copy over the files.
If someday Amazon decides to disappear one of your books, you’ll be able to copy the book back, from your backup. (You’ll need to keep that Kindle off-line, of course, so Amazon can’t delete your book again. There’s also an out chance that future Kindles will contain software that recognizes banned books and prevents your from reading them. In that case, you’ll need to convert your books from Kindle format (the .kfx extension) to something portable, like pdf. There’s a tool to do that: it’s called Calibre, and I’ll leave it to you to read about it if you’re interested.)
Nah, see, you take it off your Kindle, you decrypt it using Calibre, then you save the decrypted copy forever.
Even better, fuck Amazon and paying them, pirate it from libgen.is and do your part for “information wants to be free!”