Earlier quoted context omitted.
Wait, wut? How would they stop me from adding new book to my kindle? I can just plug into USB and load directly right?
No you can't do that on a kindle. They have a "send to kindle" feature that allows you to add non-Amazon purchased ebooks to your library. But that requires support from the backend (and an internet connection). I'm assuming send to kindle will no longer be supported on these older devices.
The big problem is that Amazon no longer allows you to download books from their site to your desktop, so you have no way to actually get a purchased book and send it to the kindle even over USB. However, if you buy non-DRM books from other book sellers you won't have this problem.