I understand that reaction but this is a director who kills Hitler in a Paris theatre. He's taking the suspension of disbelief of movies and calling its bluff. I like that and play along.
Tarantino is playing around with the whole idea of what a movie is the way Godard did. Typically filmmakers who do that run the risk of being called pretentious, particularly European ones, by people who feel threatened when a movie isn't the same paint by numbers stuff everybody does. QT is doing it in a very American way, not at all elitist, but really fun, and tremendously popular. The last really top notch filmmaker who had something new to say about filmmaking and was also popular was Hitchcock.
I appreciate the balls is takes to change the ending of the Manson murders, particularly in a place that takes itself as seriously as Hollywood and a generation that has utterly no sense of humor about itself in the Boomers. Yet the movie isn't a middle finger. It's still affectionate. The meandering you didn't like in the first half of the movie is exactly why it works -- it's getting you to dissolve into the scene. The whole sequence with Tate giddily going to the movie she's in, and the sequences of the characters just driving aimlessly in the city, are the movie's soul. The plot -- the action -- doesn't matter at all. The whole "a story has a beginning, middle, and end" is not important to what QT is doing so he just chucks it.