[mobama]

I have no experience with Kindles, so what follows is just a conjecture.

When you buy Amazon/Kindle e-books, they remain in the cloud. Downloading them is optional. After your e-reading device gets stolen, you can still access the purchased e-books in another device via Kindle app. If this is right, your Amazon/Kindle e-books should be still accessible on e.g. smartphone and you can optionally download them to upload/sideload on some different device.

If you have no plans buying more e-books from Amazon/Kindle, there is no reason for you to stick to a Kindle e-reader either. All other e-readers can read mobi files, epub files, pdfs, docs, html, and more. Your choice is wide. The only thing the other e-readers miss is a Kindle app to buy e-books from Amazon (except that Android-powered e-readers can have the Kindle app available, because there is a Kindle app for Android, but I have heard that the Kindle app behaves a bit wonky on Android and is not optimized for eink).

Should the desire arise to buy more e-books from Amazon/Kindle, you can do so via the Kindle app on the smartphone. If I have understood correctly, it is possible to download those purchases as concrete files to transfer on any other device you may have. If downloading is not possible, I see no reason to pay for it in the first place because there would be no concrete thing I'm paying for.

As for me, sideloading concrete mobi, epub, pdf, etc. files on my devices is the norm. I rarely pay for an e-book. The internet is full of free files to read. This means I am also free to pick the device for reading, as big or as small or as cheap as I want, whatever brand I want, as long as they have a microSD slot so I can stick my e-library there or a cable connection so I can transfer the files.