With an old Kindle, it was trivially easy to rip Kindle books to retail-quality epubs.

Heh, yes. The encryption key for their old DRM was derivated from the Kindle’s serial number. Wanted to crack your library? Go to the webpage, download them all there (yes, that’s now blocked, too) open them all in calibre and use a plugin to deactivate the DRM (trivially to find back then). Enter your serial number and you’ve freed your whole library.