As I understand it, Fox will still get the distributor’s cut for I–VI for any release before 2020 (or thereabouts, whenever their deal ends). It’s just that the SE is the only cut they have access to to release unilaterally, and in the absence of new material from Lucasfilm, that’s what they have to release to make money from SW.
And Disney owns Lucasfilm. Makes sense for them to not give FOX a new material they may have.
Oh man, what if we get the theatrical cuts with the truncated new Lucasfilm opening. Oh god.