The Star Wars sequel trilogy got more wrong than it did right. That's not to say the entire trilogy was a total failure. It introduced some interesting new characters and deepened the galaxy's lore. However people feel about the individual movies, each one still has some strong moments. As a whole, though, the trilogy doesn't work cohesively and fails to make much of an impact.
Many point to Disney's utter lack of a plan for the sequels as the studio's cardinal sin and the reason for the trilogy's failure. Without a doubt, Disney should have at least outlined a plan for their movies. It also wouldn't have been a bad idea to ask J.J. Abrams to sign on for directing all three movies before The Force Awakens was already halfway through production. That said, having a defined plan, or one director for each movie in the trilogy, wouldn't have guaranteed the sequels' success.
Another Star Wars trilogy had both of those elements. George Lucas knew more or less exactly where he was going with the prequels. He also directed all three movies. Despite that, the prequels were an absolute mess, and they failed to impress overall. Some Star Wars fans, especially the ones who were kids in 1999, love the prequels. Still, there's no denying that they don't hold a candle to the original trilogy.
Disney could have survived not having a cohesive plan, or a single director, for their Star Wars sequels. They just needed to take more cues from the original trilogy. When George Lucas directed Star Wars, he had no idea that his film would garner enough success to earn sequels. When it did, he didn't direct either of the follow-ups. Instead, Irvin Kershner took the lead on The Empire Strikes Back, and Richard Marquand directed Return of the Jedi.
Along the way, Lucas and the others made up the story as needed. They didn't necessarily have a plan for things from the beginning. The most famous example of their relative lack of planning is the Luke/Leia relationship. They didn't know the two were siblings from the get-go. That got decided along the way, which didn't make for the smoothest reveal, but it worked.
The tone of each movie in the original trilogy differs wildly. A New Hope is, well, hopeful. Then The Empire Strikes Back turns that hope on its head and presents a shockingly dark tone for what was basically a summer blockbuster. As a finale, Return of the Jedi needed to start in a dark place and work its way back to the hopeful tone of the series' beginning.
Each movie's variation is informed by what happened in the previous film. It's less variation, and more evolution. The movies respond to each other and progress in a natural fashion, despite not being planned from the beginning. Clearly, the production team considered what happened in the previous film and how those events should be incorporated into their newest project.
Disney didn't do that with the sequels. They had Rian Johnson begin writing The Last Jedi while The Force Awakens finished production. Johnson had J.J. Abrams's script to work with, but he couldn't really know how The Force Awakens would flow and feel, or how that feel should influence his own film. Then, when Abrams returned to film The Rise of Skywalker, neither he nor the studio looked to The Last Jedi for inspiration. Instead of following the story, they attempted to follow fans' reactions to the story. Because of that, The Rise of Skywalker feels forced, tacked-on and out of place with the rest of the sequel trilogy. Johnson's film doesn't quite fit into place, but Abrams's finale misses the mark entirely.
With the sequels, Disney went for variation instead of evolution. They were constantly targeting the films towards the audience, as opposed to bringing the audience along for a great story. When a series evolves naturally, like the original trilogy did, the variations between films help each individual movie stand on it's own. That's why the debate between Empire or Jedi being the best Star Wars sequel is still so lively and enjoyable. When a series is forced through variations in an attempt to please an imaginary audience, each individual film reinforces how out of place it is compared to the others.
Disney's trilogy could have survived its lack of a plan. The Star Wars franchise has done that before. Unfortunately, Disney didn't really have an interest in telling a Star Wars story. They just had an interest in getting audiences to buy tickets to their films. The original Star Wars trilogy is a masterclass in letting an evolving story shift its tone through multiple films. When Disney embarks on their next Star Wars trilogy, they should give the original a quick re-watch and absorb what it has to teach.