I have a pet theory about the meaning of The Last Jedi that has nothing to do with whether Luke of Rey survive the movie. It would also give Lucasfilm the opportunity to really disassociate from the old EU and take the franchise in a new direction.
Even if Luke, Rey, and Ben all survive the sequel trilogy, Luke will still be the last Jedi. The good guys will win the day in Episode IX, but the Jedi Order will die with Luke.
The Luke we see in The Force Awakens tried to rebuild the Jedi Order in the image of Obi-Wan and Yoda, but realized that he was taking the wrong path. The Jedi were so staid and beholden to tradition that they didn’t see a Sith right under their collective noses until it was too late. Luke followed their ways and was duped in the same way by his nephew and Snoke. To make matters worse, the “Jedi” name doesn’t have the standing it once had. Much of the galaxy has already forgotten them. Those who still remember the Jedi resent them because of their role in the Clone Wars, the Emperor’s propaganda, and/or because Vader himself was a Jedi.
Instead of trying to rebuild again, Luke laid low and gathered information on the founding of the Jedi in hopes of influencing a new, but different order. That is where Rey comes in.
Rey’s role in the Star Wars universe will not be as a Jedi. Either Rey will be the founder of that new order, or she will be the “new hope” of some other order (perhaps built in secret by some other ex-Jedis who might still be running around, wink wink).
One way or another, the Star Wars galaxy will go off in a brand new direction after Episode IX, with this new order taking up the cause of “peace and justice in the galaxy” from the Jedi.
One of the few threads that’s tied the post-Disney acquisition Star Wars projects together has been the introduction of other groups that either worship or use the Force, such as the Church of the Force from The Force Awakens and the Guardians of the Whills in Rogue One. Yes, this is just responsible worldbuilding, but it could also be a sign that they are building an ecosystem of Force users who could contribute to a new order in the sequel trilogy.
Ending the Jedi Order with Luke would also allow Lucasfilm to completely throw off the shackles of the “Legends” EU, where the New Jedi Order was locked in an cycle of rising, falling, and triumphantly returning over and over again for 150 years after Return of the Jedi. Although they’ve already thrown out the stories from a canon standpoint, that won’t stop fans from holding up the new stuff and comparing it to what came before.
If the sequel trilogy ends with the founding of a new order, it opens up the universe to a whole new realm of stories and worldbuilding. The Jedi could still be visited in prequels, but the future of the Star Wars universe would belong to something entirely new.