At least they still exist somewhere, albeit hidden. In the early days of television, the BBC routinely taped over episodes, even of really popular shows, because the tapes were expensive.
Example: https://en.wikipedia.org/wiki/Doctor_Who_missing_episodes
Every now and then a box of old VHS recordings turns up in someone’s shed or attic, and can be added back in to the archive. https://www.bbc.co.uk/news/uk-england-leicestershire-67218634
Yes, Kindle does now support epub. Annoyingly, it no longer supports mobi. I mean, I can still read all the mobi files on my ancient Kindle Keyboard and on the Kindle phone app, but any new book files have to be in epub format. I have a massive amount of mobi files, now have to convert them with Caliber if I want to add them to my Kindle.
I mostly use the ReadEra app on my phone, it does pretty much everything. My favourite feature is that you can control screen brightness by touch while reading, without ferreting around in settings.