It is building to the Tate murder . . . hence why the climax is
And if you consider the events of the end of the film to be a "footnote," than what do you think the climax is?
WARNING: spoilers below
the attack by the Family. Polanski is out of town, she is pregnant, the group eats dinner at the Mexican restaurant, etc. The murders are clearly intended as the climax of the film-it's why the movie gives us sequences with Tate, makes Tate Rick's neighbor, and also includes Cliff crossing paths with the Family. The whole thing is leading to a convergence at the end of the film--which historically speaking would be the murders. There's even an "ironic" music cue with "Out of Time" playing over Tate's character. Then the actual event is swapped out for the Hollywood version where the Family ends up at the house next door.
And if you consider the events of the end of the film to be a "footnote," than what do you think the climax is?
The film makes a deliberate choice to never even feel like manson's followers are any kind of threat to anybody during the climax. We know all well that brad pitt is coming back to the house after rick dalton cursed at the followers. We know that he is a badass. We know that he kicked the ass of a big muscular manson follower early on. So we know that he is fully capable to taking care of them. The deaths were played as farce.
So the actual climax is not the murders but tate inviting rick dalton into the house.