The working class whites who populated American cities were leaving or being forced out due to civilization's decline. Charlton Heston played the 'last real American man' in tons of '70s films. Travolta's "Tony Manero" character is the last metropolitan working class kid. His parents fight, his dad is out of work, his priest brother leaves the Church, his friends are going nowhere, his romantic interest is an annoying short skank or an annoying "thinks she's bettah than everybody in Brooklyn" chick (neither are hot), and he works in a hardware store. Integration and the immigration flood were already affecting Manero's crowd as evident by the gang they fight and the dance contest, creating a tension in daily life. His one time to shine is a few hours on Friday and Saturday night. King of the disco!
No matter how high he gets on the weekend, everything reminds him that life sucks otherwise. The chick who thinks she is better had to screw a guy in the office to learn the ropes. The skank he kind of dated ends up being a sort of sexual assault victim by his friends. His other buddy dies in what could be an accident or suicide, leaving behind a pregnant girlfriend. Tony's other buddies are even more lost than him; at least he can dance and has a job. The city is getting worse, and no one has a future because their present sucks. Looking ahead, that entire sphere of people will move or become swallowed up by the destruction of the '80s crack wars and stirrings of offshoring and immigration labor replacement. They were losers in '77, and they lost later, becoming virtually extinct.
Tony Manero and his kind wouldn't be in NYC or another major American city today. Their jobs are replaced by imported labor. The government city political machine has given cushy municipal jobs to minorities. Where they could afford to live, they don't want to live for fear of their life. The wealthy whites have sanitized the city for their enjoyment, militarizing police forces to use against threats. Saturday Night Fever is the swan song for the poor urban white class. If DeBlasio destroys New York, it will not be immediate event due to all of the money there. Whatever the outcome, they won't have Tony Maneros to kick around anymore.