Having said that, what if they make the whole point of the film to be "we need to restore the real timeline" and introduce that plot point early on in the film. If that becomes the primary mission of this film's characters , then restoring the timeline becomes the purpose of this film (in-film purpose). That, and the introduction of these characters to a new group of fans (out-of-film purpose).
That's a very, very weak driving motive for the members of the audience who aren't hard-core fans, though.
"Back To The Future" isn't about whether 1985 will turn out to be a proper 1985. It's about what will happen to Marty and his family if it doesn't - and by-the-by, the audience actually lives in the present (circa 1985) and therefore has at least that much investment in the "BTTF Universe."
The deal with the new Trek is: this guy Nero killed Kirk's parents. Blows up a planet. Is about to blow up Earth.
And we're assuming that a reasonable percentage of the audience cares about as much about seeing Kirk and Spock get together and save the day as they do about seeing Clark Kent turn into Superman or Peter Parker swinging from skyscrapers, just because they're Kirk and Spock and Spider-Man.
Complicate what has to matter to the audience much more than that and it's a lost cause.