Did QT forget to put a story into this one? (spoilers)

Part of it is the development and change the characters underwent, but the main thrust of the film isn't how they changed, but how they related to one another, and how they were, or were not, able to fit into a world that had changed around them.

Cliff Booth (Brad Pitt) is the real deal. He's the genuine embodiment of the John Wayne archetype that Rick Dalton (Leonardo DiCaprio) plays in his films, yet his life pales in comparison to that of Dalton's. He spends his days running errands for Dalton, constantly reassuring him that he's worthwhile, but lives in a trailer behind a drive-in while Dalton lives in luxury. Dalton got famous pretending to be Booth onscreen.

Throughout the film we see Booth effortlessly navigating dangerous situations. Whether he is casually leaping onto a rooftop to fix an antenna or talking smack to Bruce Lee then backing it up with his fists, he's a real life cowboy. His scene at Spahn Ranch is basically a real life Western. He shows up, confronts the band of outlaws, does what he says he going to do, then turns the tables on the punk who let the air out his tire. He's a bad ass and then some.

Meanwhile, Dalton is wrought with indecision and insecurity, and only shines in a pretend Western. He has to dig deep and really channel a character, and focus on the fact that he's acting. He cries over the approval of an 8-year-old girl. After his scene, in which he's compared to another famously overwrought fellow, Hamlet, he basks in congratulations for being tough in the moment of fake danger.

Then comes the end, where Booth does all the fighting, gets stabbed, kills two of Manson's clan outright, and mortally wounds the third, who staggers into the pool and would have drowned anyway, when Dalton, in an over-the-top fake Hollywood manner, as opposed to Pitt doing it with his fists and his faithful dog, burns her to death with a flamethrower. And then, of course, Dalton is given all the credit. He ends up relating his heroic tale to Sharon Tate and her friends while Booth is being taken to the hospital, and the film ends with him being granted entrance to the New Hollywood that had previously shut him out, and leaving behind the Old Hollywood, and the best friend who gave him access to it.

By this, I don't mean he was personally abandoning his friend. I mean that as Booth was being driven away in an ambulance, having suffered from an injury that would almost certainly end his career as a stuntman, Dalton was finally being allowed "behind the Green Door," and was on the cusp of starting a new acting career, one that likely wouldn't require a stuntman. He'd still be buddies with Booth, but the days of "them against the world" are over, and Booth will go back to his meager life in a trailer, while Dalton will live among the New Hollywood elite.

There's also a clear implication that moving forward, Dalton is going to be performing in more modern roles. No more westerns or retro action flicks that require a stuntman, but rather films like Polanski made-- psychological thrillers, crime noir dramas— films that require the acting chops he was able to channel in the earlier scene, with the child.

By 1969 the rugged leading man days of Hollywood have wound down, and the meek anti-hero days have dawned. The leading men are no longer of the archetype from which Dalton's image was built. Suddenly hippie types like Kristoffer Tabori, Michael Sarrazin, and Chris Jones are getting leading roles. The hippie children of Dalton-esque actors are making movies, actors like Robert Walker, Jr., Peter Fonda; even Arlo Guthrie is starring in films! And even actors we today think of as tougher. or at least more staid-- Jack Nicholson and Michael Douglas come to mind-- got their start playing counter-culture, shaggy, hippie types.

The sense I got from that last scene was that between a new wife and a pass behind the Green Door into New Hollywood, Dalton no longer needs Rick the way he once did, and their friendship will never be the same. Compare this to the first time we see them together, where Dalton breaks down in tears, and needs Rick to pick him up.