Grumpy Luke is giving a sense of logic as to why Luke would abandon the galaxy in shame (as the crawl and Han talked about).
Rose And Holdo were created because Finn and Poe didn’t have distinct or fleshed out personalities. They needed to be flawed and different to each other, and a plot showing their individual weaknesses needed to see them both fail, learn, and grow. This also is why the “useless” Canto Bight sequence was incorporated.
The Knights of Ren were absent because they were a one-line throwaway concept by Abrams, and Johnson didn’t want them to be useless fighters who were introduced and subsequently killed off. He created and used the Praetorian Guards instead. Abrams would later relegate the Knights of Ren to the same shallow goons, but with less impressive choreography.
Snoke was undeveloped and used only for Kylo’s motivation and growth, so Johnson continued along that path. Since Kylo was the primary antagonist of the trilogy, Johnson allowed him to take that role more directly while still giving him a character arc.
Johnson subverted expectations because the only film preceding was a retread of the plot of the original Star Wars, but with a bigger Death Star. The one thing Abrams changed from the original Star Wars’ formula was giving the film a cliffhanger ending. This cliffhanger got in the way of two of the three main Characters having an arc. Kylo’s choice to kill Han and commit to the Dark Side completes his arc and quest in the film. Rey is still searching for belonging, and Finn is knocked out before he can take any personal action in the plot. Poe wasn’t meant to be a protagonist, and originally was set to die in the TIE crash. Because of this, Poe had no arc. TLJ had to give all of these characters arcs to compensate, and retreading past films or giving the obvious answers to shallow questions wouldn’t achieve that.
Thus, every major issue people have with The Last Jedi can be traced to choices made with The Force Awakens.