Just to clarify, you can still use the Kindle for books purchased elsewhere or rented at a library. This article is specifically a restriction on "Kindle Unlimited" which is a spotify-like subscription for books.
Without this subscription, the Kindle still allows standard one-off downloads from the Amazon store (no surprise). But it can also accept epub and mobi file formats coming from anywhere. You can plug the Kindle in with a usb cable and move files over directly like a mass storage device. Alternatively, it offers a free "wireless transfer" service where you email epub, pbf, or mobi files to a randomly generated email address unique to your device. I use the email service frequently to transfer ePub files I get elsewhere and it works flawlessly. You send the email and open the kindle and it generally shows up in your library on the device near-instantly. It's very good.
If you use Calibre to manage your ebooks, it has a "send to Kindle" button, which is just using this email wireless transfer tool under the hood. You input your unique email address into Calibre and then when you click the "Send to Kindle" button that's how it works and it works well.
As mentioned earlier you can also transfer via usb. The newest Kindles also have USB-C which was a welcome addition.
Lastly I use my Kindle with books rented at the library. You just need a library card and a "Libby" account. The integration is a first-party integration, so no surprise it works essentially natively with Kindle. You choose an ebook at your local library and read it on your Kindle. The Kindle will even show you details about when the book is due and you can return it from the Kindle. It's all very good.
I know Kobo is considered more open because its not tied to Amazon. But I am a heavy Kindle user and have never felt weighed down by it being an Amazon device. I read books from a variety of sources. I buy books (computer books usually) from independent creators and I will send ePubs to Kindle, I will read books loaned from the library and I will also sometimes buy books directly from Amazon. The Kobo gives you the same functionality for the first two, but not the ease of purchasing from Amazon. Amazon is the largest book vendor in the world, so like it or not, I find myself still buying from there, so its nice to have that option.
I admit I have never paid for the Kindle Unlimited subscription. Its mostly low-quality novels pumped out by authors who publish 3-12 books a year. It is incredibly popular among Romance Novel readers (the largest genre of published books), but there are also lots of mystery and sci-fi books on there. But these aren't top quality NYTimes Top Selling books, they are low-rate airport novels in my experience.