The more I think about it, the more I see this playing out one of two ways:
1. Ep. IX is, in fact, a segue into a new trilogy. It's designed explicitly to do that. It would be not an end, but the end of the beginning. It would still have a sense of finality to it, but it would be a finality that also introduces the seeds of the new story (e.g., the First Order consolidates its power, but is hurt badly by a blow from a rejuvenated Resistance/Rebellion, setting the stage for war on a more even playing field). For example, this could involve a major battle that results in the formation of a new counter-government, rather than just a rag-tag rebellion or resistance. This would set a stage closer to the Clone Wars than the Galactic Civil War, with battle being between to actual governments and large-scale sides in a conflict, rather than a lopsided fight between a tyrannical empire and a small rebel alliance.
2. Ep. IX tries to actually wrap everything up and in so doing, ends up trying to do WAY too much in a single film. Kylo Ren is defeated and redeemed! Rey becomes head of the new Jedi! The First Order is completely overthrown! A new Republic is established! Another superweapon is destroyed! Stop me if you've heard this one already! Basically, they just ape Return of the Jedi in a sense, and tie everything up in a bow.
The thing is, I think Rian Johnson made it really, really difficult to put a bow on this trilogy...and I don't think Disney has an incentive to do so.
With each of the two previous trilogies, Lucas himself was making the films, and Lucas himself had grown tired of making them. We've all heard the rumors that Star Wars was originally planned to be nine films by Lucas, and that he and Gary Kurtz parted ways with Gary understanding that the story was going to head in a VERY different direction (e.g., Leia isn't Luke's sister, there'd be more of a love triangle, Luke would wander off on his own, blah blah blah). But Lucas got tired of it all and just wrapped it all up with ROTJ. Then with the PT, obviously, everything is wrapped up neatly to segue into ANH (I would argue it was done way too abruptly) with ROTS. In both cases, Lucas was done making Star Wars movies for the foreseeable future.
Disney and LFL, however, have a very different attitude. They plan to tell stories in this universe for a long time. What better way to do that than to...keep the main story going? I think they open up a ton of possibilities for the future of the story if Ep. IX merely closes one major chapter of Star Wars, while opening another. I could see Ben leaving the FO and becoming more of a "grey" anti-hero. I could see Rey teaching new Jedi or teaching them to teach themselves. I could see the FO surviving but now being genuinely threatened by a new power in the form of those who are willing to fight to oppose it. Ep. IX could showcase a galaxy dividing and picking sides between the FO and the Third Republic or whathaveyou, all of which sets the stage for a subsequent trilogy. And that allows you to tell many more stories. More than you'd get with a "happily ever after" ending that we know is just going to be upended when the next trilogy is announced.
