If IX follows the story that’s been set up so far, we’ll see the trilogy end with the FO defeated to some extent, but with the clear implication that this isn’t a happy ROTJ-esque where everything is and will be great forever - quite the opposite - there’ll be a suggestion that it’s only a matter of time before the dark side rises again. And everyone will complain that they’re just setting up another trilogy, when really they’ll be making a statement about how the fight with never be over, but as long as there are people who will stand to face it, good will win out.

I haven’t read LOTR in awhile but I recall that there’s a similar mindset with the Scouring of the Shire and all that, where the idea is that defeating evil once isn’t enough, you always have to be on the lookout for it.