I partially agree with Mijin here. It kind of is a fault of the film that it chose to focus on an obscure (to some, particularly as time and distance increase to beyond America and we consider the murders happened… 50 years ago) “historical” event (I honestly don’t think the murder, even as it happened, classified as historical in any but the most mundane sense—it’s not like the moon landing or something). The film should be self-contained and not require the audience to “read up” on the actual event before hand.
Because, I mean, I got it. I wasn’t quite sure where the film was going, but I had a wiki-level understanding of the Manson Family and the Tate murders, and I figured the meandering “fading actor” storyline was all going to come together with some kind of intervention and a chance at another shot at Hollywood “redemption” for our “heroes”, but the film really took its time getting there and for me the enjoyment had to come from that: the Manson Family and the expected ending weren’t nearly “present” enough to be compelling. So for the most part, you got a long meandering movie about a failed actor, and if you knew enough you held out hope for an action scene at the end, but then if you didn’t know about the murders, there wasn’t sufficient reason to conclude they were coming (they showed Manson surveillance the house, but it still wasn’t clear enough early on that the Manson Family would be coming to our heroes’ neighborhood).
In short, I think a lack of setup and in-universe foreshadowing is a shortcoming of this film, but the main storyline is done well enough to make it watchable, even without the murder-climax. So they absolutely could have ended the film differently, but then they might as well have cut out everything about Tate and the Manson Family, too.
BT
As a random aside, there is a fine line between not enough setup/exposition for a perhaps obscure quasi-historical film, and way too much. On the flip side of the coin, there’s the 2005 film The Great Raid, which opened with a long, clunky expository narration that felt the need to inform the audience that there was his thing called WWII, and that it involved fighting between the US and Japan, and an attack on Pearl Harbor, and an attack on the Philippines, and …, and … and… and I just cut it off probably midway through the opening narration because I just like “Holy NFBSK! Just how dumb of an audience did they make this for, that they feel the need to give me a first grade history lesson before even the first scene?”
So it’s a fine line. Once Upon a Time… in Hollywood did I think “not enough” in terms of setup for a potentially obscure historical event (to some audiences—but I maintain the film overall was good enough to carry my interest personally), while I think some films do way too much, to the point of distraction, taking the time to spoon feed expository and world-building information about a world that a minimally informed audience would already possess.