It's the last Jedi because he will literally be the last 'Jedi'.

The first 6 starwars movies were about the Jedi and the Sith who are the 2 polar extreme opposites of each other.
The new starwars will focus on the inbetween. Luke probably found out after defeating the sith and still being betrayed that there is no better side and so he will be the last jedi and Rey/ whoever else will something new so neither Jedi or Sith. (same has already happened with Kylo Ren and the first Order, he isn't a Sith and neither is Snoke but has been trained in the light and darkside so he is basically both.)
Doesn't mean there will be no more lightsaber wielding good guys, they just won't explicitly follow the Jedi code or whatever anymore.