Shakespeare spoiled lots of his endings in the first few lines of his plays. That becomes part of the dramatic suspense--you know the ultimate fate of the story, but what you don't know if how things unfold, who does what, and knowing how things turn out gives you new dramatic mechanisms to use. You can use it to mislead and twist the viewer, knowing she or he expects certain things, or you can use their knowledge to play up the drama. A more banal instance of the latter is in horror films where the audience knows a monster is lurking behind a door but a character doesn't--the suspense becomes not "is there a monster behind the door?" but instead, "no, don't open that door!" and then the audience has to squirm in suspense wondering how the character will survive.

Anyway, most blockbuster movies like Star Wars are predictable. Will Luke beat the bad guys? Yes. Will he save the princess? Yes. Will he survive the ordeal? Very probably. But how does he beat the bad guys, what situations does he have to get himself out of, and how does the princess get rescued? This is the suspense structure of Star Wars. That's also one reason why it was downright shocking when Luke got his ass kicked in ESB and all the good guys lost--that's not supposed to happen!!