For me, the kerning, hyphenation, and spacing improvements in KFX are pretty big. I also like that I can choose justified or ragged right.
https://www.reddit.com/r/kindle/comments/viqxjj/heres_the_fo...
Regardless, my point was that: A - most of the books already in the library were likely enjoyed/acquired without knowledge of hypothetical improvements from Amazon rolling out a new ebook format and DRM scheme. And B - even if there is some magic that Amazon had to include in KFX to support the improvements you listed and can't be reproduced without them; I personally would not consider those or most any improvements to be worth losing ownership of books that I purchase. The most valuable part of an ebook is the text and ownership of a copy of that is what I'm paying for. It is fairly easy for me to be principaled on only buying ebooks that I know I can own a copy of due to the diverse distribution that exists for most titles. Even when I had an Oasis, I didn't purchase anything through Amazon and loaded all my books over USB.
Seems like that's how it should work, but it doesn't. Maybe that's by design or maybe it's fallout from poor choices Amazon made earlier in Kindle history. I really don't know.