Rian Johnson said that, once the title of the movie was revealed, that he knew Star Wars: The Last Jedi would be the title of the movie very early on.
Not only that, but Supreme Leader Snoke also referred to Luke by that title, so there was quite a bit of evidence to suggest that Star Wars: The Last Jedi is directly tied to Luke Skywalker.
Star Wars: The Last Jedi is written and directed by Rian Johnson and is set for release on December 15.
It didn’t help anything that many of the foreign versions of the title for Star Wars: The Last Jedi suggest that it had a plural meaning.