The big problem here is that devices can not be re-registered. It's a mean move from Amazon, and will make it difficult to re-enable extra features. However, those devices have multiple jailbreak methods available, so there's really no loss if you can take that extra step. All books are presumably still available on the kindle app / website, and because you already bought them you can pirate them.

Kindles have the best text rendering (imo), and calibre can be used to sideload books. My PW1 had stellar text rendering. My next kindle, Kindle 10 had a lower PPI but decent text rendering. I now use a PW5 and the text is flawless.

Kindle's UI does suck, though. Very slow and the keyboard is glacial. Still, page turns are zippy and it collects highlights in a central file, which is very handy.