Another potential reason for Luke to say that it's time for the Jedi to end is that he simply doesn't want to train anyone else, Rey or otherwise. We see Luke training Rey in the trailer for The Last Jedi, sure, but maybe he needed some convincing first. There's no reason to assume that the dialogue in the trailer runs in chronological order.
Some fans have speculated that Rey and Luke are related in some way, which would be a logical reason for Luke to eschew the ways of the Force—he doesn't want a repeat of the ol' "everyone I love turns to the dark side" story line. That's possible, though it would also be yet another rehash of the old "I am your father" trope. Luke's and Rey's names already imply history repeating itself. (Luke means "light," "Rey" of light.) Either Luke is no Master Yoda and he knows it, or he is consciously choosing to emulate the hermetic lifestyle of cranky old Yoda in his retirement/seclusion on Dagobah, which might actually be worse.
On the other hand, maybe he just doesn't want to see any more ghosts. Since Luke is the only one who can see the space ghosts of Obi-Wan, Yoda, and Anakin, there's reason to be optimistic about the chance of some of these spirits showing up in The Last Jedi. Could the muttered words of Darth Vader and Obi-Wan in the trailer's opening be evidence of Jedi ghosts? Perhaps Luke would prefer that there be no more Jedi because he already has enough space ghost friends. Three's company.