To start: this is not about how "The Last Jedi" fails fundamentally as a film, this is just about its characters and the choices they made with them this movie.
It's no small secret that George Lucas traded a chunk of salary for new hope in exchange for a percentage of royalties, and as such made insane amounts of money. It is easy to tell that the Ewoks from Return of the Jedi are intended for kids toy sales for children who were not yet old enough for action figures, but even at that they still serve a function and a purpose for the story.
The Star Wars franchise is almost handicapped by the scale of its' own brand because of this and I believe, based on what I have seen from TLJ, that this motivates the creative process behind the movie and has tarnished the brand.
There is a lot to go through before I get to my main point so bear with me here, or just scroll down.
Disney's decision to bring Star Wars back was obviously a money grab, and the story of the SW Universe quite clearly ends with Return of the Jedi, there are no loose ends left to tie. Luke is a Jedi Master, his father's soul is at rest, Han is with Leah, and the Empire and the Emperor have been vanquished. So in bringing back such a beloved and finished franchise, they would have to leave it in the hands of someone who is experienced in reviving big brand franchises that have both fan and mass appeal, and there was no better at this than JJ
Star Trek (2009) was very well received by both Trekkies and the general audience alike. It is not a groundbreaking film by any means but it did an amazing job of reopening up the universe to the modern audience in a way that made them feel connected to the story regardless of it also being a reboot cash grab. The story was concise and focused, there was a central villain with an obvious motivation the characters were simple but still human enough to connect to. Spock feels marginalized and is torn between two worlds, Kirk is trying to live up to his father's legacy, Bones was just Kevin Urban being himself but that is still pretty accurate, and I teared up a bit in memory of Anton Yelchin. But these were characters, not caricatures.
He continued the series with Star Trek: Into Dark, which was not as well received, it was a bit long, the characters weren't as good and the new characters added nothing to the story. It was dark and a bit depressing. But it was not a disaster, while not a good movie, it certainly wasn't bad either. There weren't many who enjoyed the film and fewer eagerly anticipated the next one. But it still grew the characters and contributed to further the plot, despite ruining the characters of Spock and making Kirk's sacrifice for Spock worth nothing, but that is beside the main point.
Star Trek Beyond, released 6 months after TFA, was the best of the 3. JJ was only a co-writer and producer, with Simon Pegg (who used the opportunity to make himself more of an action hero rather than mostly being comedic relief but that is ok because it didn't take away from the overall story) and Justin Lin as the director. The movie was incredible, the characters were once again relateable and had real motivations in a story where their actions had real consequences. Kirk was celebrating his birthday and become older than his dad was, while also feeling the drain spending so much time on missions has. He makes a rash decision and it costs the lives of most of his crew as well as the enterprise. He grows from this experience and becomes a better character because of it.
TFA went off without a hitch, a well-received film and a good one by all standards, most Star Wars fans loved it and its only real detraction was that it was too similar to the original movie. Rey, Finn, and Poe were positive additions to the universe, with unique stories and motivations that made them feel like real people. Rey was determined to figure out her past, which she thought was on Jakku, so leaving it was a big step for her character and contributed to her growth. Finn defected from the empire, a job he hates with an awful and mean boss, and is now actively fighting against it. Poe was a rouge, willing to sacrifice himself for the mission and the greater good (eg the opening scene). Rey and Finn grow as characters throughout the movie and because of Han's death, which they seem determined to avenge. Kylo was an intimidating villain, with immature emotions and an unruly power. Snoke was mystical and intriguing. After the movie, we were left waiting for the answers to questions like Who is Snoke? And who are Rey's Parents? Questions that did not need to be answered immediately so most fans were content to eagerly await the answers.
I make these points about TFA to show all of the ways TLJ fails.
Han's death has little to no impact of the plot of TLJ. Leah inst overly broke up by it and it does not influence her decisions
"Well she's a battle-hardened general and they were separated so she could easily move by it" thank you counterpoint voice in my head. While that is true, it is far less interesting and makes the movie feel more disconnected from the previous stories. Rey does not harbor a grudge against Kylo for his actions. Luke does not much mourn his friend's death, and the story moves on and our favorite scoundrel has died in vain. Rey does not grow as a character, she is not trained by Luke, the scenes on the island are weird, disconnected, and comedically awful. She her lack of training, decision making, and having no growth makes her fail as a character. She does not make many mistakes in her personal training and as such, learns nothing. Finn is a bumbling idiot, walking around the ship in a suit that is leaking water for comedic effect. He does not productively channel his anger against the empire into a positive way, he does not make strides in tearing them down. And worst of all, his attempt at sacrificing himself to help the rebellion was ruined by Rose. He learns nothing, doesn't grow, and fails as a character in this movie.
The reason I pick out these two over evil wide man child and captain "let me make military decisions myself to the destruction and death of my fellow soldiers because 'its a fleet killer'" is because Finn and Rey are the most marketable toys. A child with a Finn action figure would be heartbroken to see him die before the finale. Likewise for a child with a Rey toy, who would be devastated to see her hero join evil Kylo. But these would have been better choices to make (and appeared to be the obvious ones). Finn sacrificing himself to lead to the safe escape of the Rebels would have been a huge step forward for the story OR EVEN Rose to sacrifice herself to save Finn and Finn having to live to avenge her death and the guilt that comes with a friend sacrificing himself would have given his character GROWTH and make him interesting and more relatable again. If he dies Rey feels heartbroken and angry and uses that motivation going into IX if Rose dies Finn feels guilty and uses that motivation in IX, either way, characters grow. But they cannot make these choices because if Finn dies then they have to answer calls from angry moms who's kids are heartbroken because of it.
Another more interesting option would have been either Rey joining Kylo in an attempt to stop the war and bloodshed OR even more interesting, her and Kylo switching places, with Kylo repenting of his sin of patricide and rejoining the light side, while Rey's unquenchable thirst for knowledge leaders her into Snoke's hands and to the dark side. But these decisions cannot be made because they have to sell the "Rey is a good guy" toy.
A counter-argument to this could be that Marvel killed half their characters in 2 minutes, regardless of them obviously coming back. But the difference is that Marvel
1. knows their audience is older and thus has the more creative license over killing off characters and
2. has killed off key characters in the past.
In spite of the earlier blunders of the "The Last Jedi", if either of those changes happens fans are once again eagerly anticipating the next installment. But they didn't, and the same old status quo is old and tired.