"Return of the Jedi" was also a plot point, but nobody cared about giving that away before the movie was released. And if Luke decides that Rey will be the last jedi he ever trains, the last jedi the galaxy ever sees, would that really be a shocker? Every jedi order ends the same way: With one student going bad, slaughtering everyone else, and wreaking havoc across the galaxy. Is Disney/Star Wars just going to continue that cycle ad infinitum? Doesn't it make sense for Luke and Rey to see that pattern and resolve to break it? Maybe Luke trusts Rey to kill Kylo Ren and not to fall to the Dark side. But it makes perfect sense to me that neither he nor Rey would want to risk training any more jedi after that.