Luke Skywalker originally intended to leave Ahch-To and rejoin the Resistance with Rey in Star Wars: The Last Jedi. The pair's dynamic was described as the beating heart of the movie, as the young scavenger attempted in vain to convince the grizzled Jedi to pick up his laser sword and take on the First Order. After some tough lessons and much time together, Rey tried to shift the tide herself, leaving Luke behind on the island to spend the rest of his days in isolation.
Of course, Luke did eventually change his mind and called upon the full extent of his Force powers to astral project himself across the galaxy, sacrificing his life so the Resistance could survive. In the film, this decision appears to be spurred by Yoda's final lesson to Skywalker, but things play out differently in the official novelization. Luke was very much planning on physically leaving Ahch-To and help his fellow heroes.
In the book, this additional scene takes place after Luke reawakens the Force inside him and Rey has her experience in the Ahch-To cave. After closing himself off from the universe for so long, Skywalker finally realizes Rey was right and he is needed by her, Leia, and the Resistance. As Luke walks back to the village, he calls for Rey saying, "I'm coming with you." However, he ultimately decides not to go when he sees Kylo Ren and Rey in the midst of their final "Force Skype" talk, holding hands. Rey and Luke's biggest disagreement was over Ben Solo's fate. Luke believed him to be lost, but Rey felt he could be saved and become a valuable ally.
This is without question one of the biggest revelations in the book, though it's understandable to see why it wasn't in the movie. Frankly, the scene flows better without this extra bit. Luke's transition from feeling "relief" at the sight of the Millennium Falcon to telling Rey to leave the island is quite abrupt, and would have amounted to little more than a cruel bait-and-switch. Additionally, the moment where Luke reaches out to Leia on the meditation rock is enough to infer he's having second thoughts about his exile. That was a far more subtle indicator of his character arc's progression (he had closed himself off from the Force) and didn't hit the audience over the head. Since Luke was seeking out Rey after his reawakening, he probably had a return in mind.
It would have been fun to see Luke return to the Resistance in the flesh, though there is something awe-inspiring about the ending we got. Not only did Skywalker triumphantly show up at the last minute to save the day, his last stand on Crait was visual confirmation of how strong he grew since Return of the Jedi. The 30-year gap between the original and sequel trilogies meant we missed on seeing prime Luke use his abilities, and Episode VIII was the last chance we really had. Astral projecting, while having precedence in Star Wars literature, was never depicted onscreen before. The reveal of Luke far away on Ahch-To, exerting himself to save his friends, is far more poignant than watching Luke wreck shop with a lightsaber a la Darth Vader in Rogue One.
Source - Star Wars: The Last Jedi Novelization
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