If there's a JL-like effect on Episode IX and it actually far-underperforms, I will be shocked.
It's a totally different situation, imo. The starting points for these respective franchises are The Force Awakens and Man of Steel. One is one of the biggest hits of this century and a reintroduction of Star Wars to a new generation and the other is a modestly successful, but underperforming reboot of Superman.
One of these franchises is clearly more popular with kids and families than the other, and that makes a huge difference alone.
No denying that TLJ is causing a sharp fan divide, but I'm not sure how much that's going to actually effect the box office for the next film. I do think the Star Wars-fatigue factor is something Disney should keep an eye on. But the characters of Rey and Kylo have massive popularity with a new generation of fans, and I have little doubt people will still flock to the theaters in a couple of years to see the conclusion of their story.
Obviously, I could be wrong. I just haven't seen anything drastic enough with the box office of this film yet to suggest that this franchise is in actual trouble. Nobody in their right mind was expecting anything near TFA, which was a once in a generation type of phenomenon. Besides, there's historical precedent for the middle chapter in Star Wars trilogies to have a bit of a dip. DC/WB would kill right now to have their "underperformer" on track to cruise past 1 billion and likely end up somewhere near 1.3-4 billion.