I think ‘The Last Jedi’ refers to Luke abandoning the conceptual ideals of the Jedi, which is what led to their downfall in the first place (and possibly what led to his failure with Ben and the loss of his students).
Luke will be the Last Jedi because he abandons those ideals when he trains Rey. Rey’s training will embrace both the dark and the light, but only in an attempt to do what is right and just. We saw him sort of embrace that in ROTJ, and maybe that scared him, but after losing all of his students and reexamineing the ideals of past Jedi, he realizes that suppressing the dark side isn’t the best way to go. So he teaches Rey to embrace love, and to accept possesion and attachment as a natural part of life and the force.
So he, in the end, truly is the last Jedi. That would be a good way to both acknowledge the prequels, but also distance itself from them.
But I do think he will at least make it to Episode IX
I would LOVE this!
Resisting the darkness is a conflict, as well as it seems that for Kylo it is to resist the Light. Supressing that renouncement in both cases implies supressing the implicit conflict in those characters, whose development is strictly attached to how they learn to cope with their special conditions (being orphans, handling the force, etc.). If the Force is presented as an harmonic whole, a unity of both dark and light, then we might be left with just the story of some people with superpowers among regular people.
While I do think too that it would be nice and different to see in a Star Wars movie, the further implications of a superpowered person with a flexible moral compass doesn’t seem hollywoodsy, nor Campbellian enough to have a feasible development in IX.
The acceptance of a dark and a light in balance seems to be the meaning of elder wiseness in Star Wars, in Yoda and in Obi Wan (whose ridiculously just-because Jedi Code was responsible for the destruction of the Order), in Vader, perhaps even in this Sequel Trilogy Luke. But I don’t see it being appliable to an idealistic teen/young Rey that by her sole age should be the force that tries to fix the wretched world.