I think the biggest concern for me is if Snoke just happened to pop up in between 6 and 7, what is stopping someone just like Snoke from popping up after 9’s “happy ending” and restarting this thing again and again?
If they had explained Snoke was tied to Palpatine, either a former apprentice or Palpatine himself, that would mean once Palpatine dies in 9 we can safely say the Sith are finished. That is why I hope we get an explanation for Snoke.