The thing is, for most people, Star Trek IS TOS. The fact that those characters are coming back, even if they're being played by entirely different people, guarantees a big opening weekend, at least. If the movie is at the very least decent, it's going to do really, really well.
The truth is, 90% of moviegores don't give a shit about Benjamin Sisko or the chick from Voyager. They know "He's dead, Jim!" "Beam me up, Scotty!" "Where can we find the nuclear wessels?" That's what they want from Trek, and that's what they're gonna get with this new movie.