Rian Johnson, the director of The Last Jedi, did an interview with the New York Times in which he revealed who the title to the new Star Wars film refers to.
It’s in the opening crawl of “The Force Awakens.” Luke Skywalker, right now, is the last Jedi. There’s always wiggle room in these movies — everything is from a certain point of view — but coming into our story, he is the actual last of the Jedi. And he’s removed himself and is alone on this island, for reasons unknown.
So the title refers to that point in time, and does not necessarily mean that Luke will remain the last, or only, Jedi in the universe. But even if he is, the saga will still go on. Uproxx has an opinion piece that posits that the survival of the Jedi order is not as important to the Star Wars saga as you might think.
The future of the Jedi may be in question, but even if the Order ends with Luke Skywalker, the Force will remain. Both Rogue One: A Star Wars Story and the final season of the Star Wars: The Clone Wars set up the idea that the Jedi and the Sith are merely the most active sects of an ancient religion that predates the invention of space travel in a galaxy far, far away. Whatever deity or universe-effecting energy permeates the Star Wars universe, it doesn’t need lightsaber-wielding humanoids to keep existing. In fact, based on other quotes Johnson gave the Times in his interview, Lucasfilm is inching away from extreme Light Side/Dark Side dichotomy and towards a “gray area.”
Both articles go on to describe the psychology between the forces (pardon me) represented by Kylo and Rey, which could carry at least a couple more movies. The overarching themes here completely ignore midi-chlorians, which are best forgotten. We can just put that in the files as a theory that Qui-Gon Jinn was completely wrong about.