All of that is technically correct, however my non-Amazon readers (Sony, Nook, ...) reached the end of their useful life when their batteries died or their screens broke. All of them were "unsupported" at that stage (3-4 years after announcement), so not much to do about it. With that, I have a very old Kindle (7 years or so) that is still working -- with "Amazon's blessing" of course.

Any reader will turn into a brick one day. What matters is what you're getting before that point. For me, I'd rather use Send-to-Kindle and never bother with SD cards again. Naturally, YMMV.