Actually for the first 2 hours the audience fully expects that the Manson klan of Leftists WILL enter the house and kill the guests.
Tarentino flipped the audience expectation and gave us a happy ending.
Quite a few films begin with “Once Upon A Time” including the Westerns and Star Wars but also the French Beauty and Beast long before that. And filmmakers build off of the traditions of others. Cocteau’s Beauty and The Beast begins with a preamble (as does Star Wars and Tarentino puts the line in the title of his movie):
“Children believe what we tell them. They have complete faith in us. They believe that a rose plucked from a garden can plunge a family into conflict. They believe that the hands of a human beast will smoke when he slays a victim, and that this will cause him shame when a young maiden takes up residence in his home. They believe a thousand other simple things.
I ask of you a little of this childlike sympathy and, to bring us luck, let me speak four truly magic words, childhood’s ‘Open Sesame’: ‘Once upon a time...’”
Tarentino makes it quite clear with the end message.
I feel very fortunate to have seen Once Upon a Time in Hollywood in a theater and without having any clue it would end the way it did. Tarantino is a genius.