The dark side will always exist. Evil will always exist. And in times of peace dark forces will eventually rise again. But that doesn’t mean there is no point in fighting against that evil. So in my mind, that is why Rey proclaims herself a Jedi at the end of the film. Calling herself a Jedi is an act of rebellion against this absurd cycle. It’s saying, “I don’t care if I can’t stop the cycle, I’m going to resist it regardless”. She’ll fight for good because it’s worth fighting for, no matter how impermanent peace is. And that not only speaks to the issue of the Sith and the Jedi, but also with the New Republic/First Order.
But regardless, the movie is open ended enough to make the audience wonder if the cycle is broken, or has changed the state of the galaxy for good. Maybe it has, but whether or not evil will rise again doesn’t matter, because Rey and our heroes have accepted the fact that they will face whatever threats may rise, because it’s the right thing to do.
Does any of that make sense?
I think that’s good, as long as we make it absolutely, 100%, unambiguously clear that Palpatine will NOT be coming back. Evil in general always returning to threaten the galaxy is a nice bit of realism, but the EXACT SAME GUY coming back every 30 years to ensure endless OT repeats ad infinitum is downright cynical. It’s possible for Rey to get rid of Palpatine for good without all evil in the galaxy necessarily being destroyed (although I’m not quite sure how you’d convey that with the footage we have).