I can't see any real motivation for it either. The sequel trilogy was so safe over all, that the state of the universe is almost identical post episode 6 and 9. The only thing a retcon would accomplish for mainline media is free up some specific characters tied to actors who are way too old to play them anymore or literally passed away (please tell me there will not be a de-aged Mark Hamill film Disney). If you want to do Thrawn movies you can just do it post 9, nothing really changes. If remotely true I feel like this is a corrupted version of "we're going to re-canonize the EU stuff as an alternate timeline as a mea culpa fan appeasement?"