Luke in exile because he failed at training Han and Leia’s child was straight from Lucas’ outline he sold to Disney in 2012. The biggest difference was, in his outline, that would have been part of the story told in VII, rather than something that happened between VI and VII. Luke would exile himself in VII after a character based on Darth Talon literally seduced Han and Leia’s kid to the dark side with Luke failing at preventing this, then Luke would have to be found by “Kira” (later Rey) in VIII.
Lucas hired Arndt to write VII, but he had a problem with Luke in VII taking too much focus away from the new leads. It was Abrams’ and Kasdan’s idea to make finding Luke the MacGuffin of VII, pushing his appearance to the ending to avoid overshadowing the new leads, which also meant that the fall of Ben Solo had to happen off-screen before VII.
There’s more of Lucas’ ST in the final product than many are claiming, but his VII pretty much got split between the final version of VII and VIII.
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