It seemed as though TFA was setting up a mystery box to eventually explain why it all came so easily and that maybe it was a lineage thing (which I don't particularly like) but then that was discarded by Rian Johnson.
I don't think its necessarily fair to suggest Johnson discarded anything. If he did anything, he punted to Episode IX. Which honestly makes sense because it is a big mystery that, while minimized in TLJ, is a throughline for the trilogy: Who is Rey? Why does she have these Force powers? As for anything regarding lineage, even from the outset of TFA, Rey is set up as a no one with parents who were no one. I think ultimately, the mirror cave scene is going to mean a lot more once the trilogy is complete.