What are some "killer" applications that would tempt the casual Kindle user to jailbreak the device? I can see someone has ported syncthing [1], which could be convenient for syncing the contents of the device. But probably still too much work compared to using e.g. Calibre and a USB cable a few times per year. [1] https://syncthing.net/
The smallest margin on Kindle Scribe is too large. Changing text justification is not possible. Books cannot be uploaded directly with a USB cable without additional software (calibre). Gesture configuration (tap, two-finger tap, etc). Koreader can do everything you want and is significantly better than the stock reader. Unlike other vendors, kindle requires root to install it.
A reader with native epub support is a good feature. With the stock reader, Calibre needs to do an epub -> mobi conversion (works well, but still...).