That's why the more they get into Snoke's history, the better. I think it's fine that Rey is training to be a Jedi in EP VIII but I think EP VIII needs to focus on the past of Kylo and Luke's new order and definitely Snoke because I left the movie thinking, jeez there are so many things unanswered from TFA and that's why I thought the plot for TFA was so basic evermore so than ANH.
I know there's speculation all over of how fans think how this new generation came to be, but there is no proof yet. And until we physically see it in a movie, I'm not convinced of anything.
Hopefully after Rebels we get a series set between ROTJ and TFA, although unlikely since theres about 30 years worth of events to cover.