Almost cried at the end of this...*spoilers*
A lot of people who saw this movie are shrugging their shoulders going, "I don't get it." But having similar sensibilities to QT, I kind of get it.
I don't know how to explain this, but sometimes if you're really into something (art, music, fashion, politics, etc.), you become sentimental about certain periods in history when you felt that this thing had hit its peak and there was nowhere to go but up. But then it turns out that just when it hit its peak, something horribly tragic cuts it short and changes the course of that thing forever. As a result, you wind up obsessing about "what could have been" to the point where you keep wishing you could go back in time and change things.
For example, if you're really into progressive politics, it might really sting that JFK, RFK, MLK and Malcolm X were killed, to the point where you can't help but thinking, "Man, imagine what our country would be like if they hadn't died? Imagine if the worst that happened was that they were grazed," or, "What if someone had stopped the killers in time?"
Or maybe you're into music or comedy or whatever, and an idol of yours died tragically (for example, John Lennon, John Belushi or Kurt Cobain), and their death had such impact that you felt as if music or comedy "died" or was changed forever. And all you do is keep thinking, "Oh, man, if only John Lennon had been winged," or, "Belushi never took that hit of coke."
When I watched Once Upon a Time in Hollywood, this is the feeling I got from it. It was QT engaging in a wish fulfillment fantasy in which he was thinking, "What if Sharon Tate had been saved? What if the Manson family had been stopped? Then Old Hollywood would've never died, and it would've never lost its innocence."
I know that doesn't make sense but the Tate murders were kind of like the 9/11 of Hollywood at the time. (Please don't yell at me; I'm from NYC.) That incident was a complete game changer. Up until that point, everyone had romanticized and glamorized Hollywood as this totally fun, beautiful place where you could fulfill your dreams, people had fun and nothing bad could ever happen. But then when the Manson murders happened, the balloon was popped, and old, glamorous Hollywood was replaced by this uglier, seedier Hollywood.
This is what I felt as I was watching the movie. Even though there were a lot of crude jokes and everything, it was a very sentimental, kind of bittersweet movie where he was lamenting the loss of Old Hollywood and creating this wish fulfillment fantasy/fairy tale where it never died. Sharon Tate and her friends get to live long and happy lives, and Old Hollywood (represented by Rick Dalton) gets to carry on.