The only reason the ST has a chance of feeling like its narratively cohesive is that Abrams is returning. Had Trevorrow headed IX as was originally intended, it would have ended in disaster.
I always felt that when Colin Trevorrow was announced to direct Episode IX, it was an abrupt decision, since Jurassic World had come out and was very successful. In an interview on Collider’s Jedi Council, SNL cast member, Bobby Moynihan said that Daisy Ridley asked Trevorrow what happens to Rey in IX. He took her to the back room, and she came out crying. The story may have changed since Trevorrow was on board, but I do think that bringing J.J. Abrams back was a smart choice. I loved what he did with The Force Awakens, but I also loved what Rian Johnson did with The Last Jedi.
To put it simply, Abrams helped create the world and new characters for the sequel trilogy. He and Lawrence Kasdan (as well as keeping some of Michael Arndt’s material he created before leaving, because he was credited in the final film) wrote the story for VII. When Rian Johnson took over, he helped write the story for VIII, yet still continuing some of the story arcs set up by Abrams and Kasdan. While the story went a different direction from what J.J. may have done, he still continued the important arcs from what J.J. and Kasdan set up. Luke was always going to be a hermit, refusing to come back into action (an old idea George Lucas had for his sequel trilogy), as well as the relationship between Kylo Ren and Rey blossoming into a potential romance. What J.J. and Chris Terrio are going to do with IX is bring the trilogy to a close by continuing where Rian left off, and ending the story arcs where Abrams began off with.
The best thing to do is to tell the best story moving forward in the Star Wars saga, not bring fan service to the big screen, such as old Luke Skywalker with a lightsaber taking down the First Order. This is something I wanted at one point during the two year gap in between the releases of The Force Awakens and The Last Jedi, but I am happy that they did not do that, as it would be out of character for the story set up for Luke. Even Mark Hamill agreed with the decision by the end of making VIII. He disagreed at first, but soon realized that the writers knew better and loved the ending for his character.
I am very excited for Episode IX, and I cannot wait to see what happens between Rey and Kylo Ren for the end of the sequel trilogy!