Star Wars is about an entire galaxy of characters, and a conflict/war can be started for any number of reasons by any number of characters. Sith are not the only ones who can start a war. Sith are not the only ones who can cause conflict. We already spent 6 consecutive movies in which the Sith were the central villains, causing the central conflict. That part of the story is finished. Even if there are other Sith somewhere in the galaxy, there's no reason to keep making more movies about them - because that's redundant to the point of being boring. But even without the redundancy issue, my point is in my first statement: There's a whole galaxy of characters, and a whole galaxy of reasons to make more stories/movies within Star Wars. It's not all just Sith-VS-Jedi.

Think of it the way the first movie was made. The very first movie (A New Hope) wasn't even about Sith-VS-Jedi; that was just a peripheral idea meant to motivate Luke's character. It wasn't the central theme.

Even as "Empire Strikes Back" and "Return of the Jedi" expanded on the Sith-VS-Jedi theme, it didn't make it the central focus. The central focus was about a young man trying to redeem his father and save the galaxy at the same time. Sith-VS-Jedi was still just a peripheral idea meant to strengthen the characters.

Episodes 1 through 3 focus very deeply on Sith-VS-Jedi, and that's okay; because they are the backstory for the original trilogy. But that whole story ends in a definitive way with "Return of the Jedi". The Sith are finally defeated, Anakin is redeemed, Luke becomes a man, and the galaxy is saved. 6 movies that show us how and why the Sith might take over the galaxy. 6 movies that show us how and why the Sith might be defeated. It's done. We've seen it. It's time to move onto something else, a new villain, a new conflict that causes a new war.