I have a kindle for more than a decade and I never bought a book on amazon or anywhere else. I use it as a reader, it's never been connected to internet
Note that if you ever do, it'll delete stuff you've added to it. I connect mine to the net every week or so (I like the translate feature, and use some pocket-to-kindle thing), but if I ever leave it for over a month or so it deletes my books.
(Fortunately it's easy to get them back from calibre, but very annoying.)
Where do you source your books? I love my Kindle as an ereader and I get the books from my library, which sends them to my Kindle via Amazon. So I am connected to the internet.
I read classics and old books, the authors are long dead so I don't feel bad about downloading these. If you mostly read niche or brand new books it probably isn't as good. You can find a lot on project Gutenberg, archive.org, &c.
Same. The blog says, "Download & Transfer via USB" option will no longer be available, but "You can continue to sideload e-books on your Kindle via USB cable".
What is the difference? How is sideloading different from normal USB Transfer?
You can transfer books you make or download elsewhere to the reader over USB but Amazon is no longer going to let you download the files from them. Amazon books will only transfer directly to your reader over Wi-Fi.