This is a major theme in The Illusionist (2010) as nobody wants to see stage magicians anymore and nobody believes in magic.
Kung Fu Panda 2 has this as a theme of the story as the advent of the cannon threatens to make kung fu irrelevant and useless in battle. However, Po averts this spectacularly by discovering an effective kung fu Catch and Return technique to defeat cannon fire, making kung fu still a vital skill in battle that can counter artillery. Furthermore, Po rather casually makes it clear that he can teach his friends this technique, which means it will be spreading throughout China in due time.
The ending of Song of the Sea. Saorise regains her coat and sings her song, ushering all of Ireland's fair folk across the sea to Tír na nÓg. She also sacrifices her selkie coat to remain on Earth with her human family, taking with it the last remaining bond to the fairy world.
Cars 3: The NASCAR-style stock racers like McQueen are quickly being sidelined by new GT3/Le Mans-style racers such as Jackson Storm.
Apocalypto: The Mayan civilization is on its last legs, and the Spaniards arrive to the Americas.
The Artist is set during the twilight of silent films and the emergence of talkies.
The lightning strike to the clock tower in Back to the Future arguably represents the beginning of the eventual decline and decay of Hill Valley's town square. Or, in a broader sense, it perhaps symbolizes the end of America's 1950s post-war boom.
It's largely only subtext in the film, but the novelization of Robert Zemeckis' Beowulf makes it clear that for the Norse people the dawning of the age of Christianity means the end of the age of myths and legends, something which Beowulf greatly resents.
Boogie Nights deals with end the close-knitted 70s, 80s porn producing subculture and the rise of the open market porn industry. Little Bill, played by William H. Macy, appears in the 70s scenes, frequently complaining about his wife. However, at a New Year's Eve party, ushering in 1980, Little Bill shoots his wife dead, says "Happy New Year" to the shocked partygoers, and then shoots himself in front of them. The scene sets the tone for the grim, uncertain 1980s after the carefree, hedonistic 1970s.
Sam: The town will never be the same. After the Tangiers, the big corporations took it all over. Today, it looks like Disneyland. And while the kids play cardboard pirates, Mommy and Daddy drop the house payments and Junior's college money on the poker slots. In the old days, dealers knew your name, what you drank, what you played. Today, it's like checkin' into an airport. And if you order room service, you're lucky if you get it by Thursday. Today, it's all gone. You get a whale show up with four million in a suitcase, and some twenty-five-year-old hotel school kid is gonna want his Social Security Number. After the Teamsters got knocked out of the box, the corporations tore down practically every one of the old casinos. And where did the money come from to rebuild the pyramids? Junk bonds.
Downfall is about the end of World War II for the Germans, the end of the Nazi era, and the crashing down of the fantasy world that the Nazis had constructed as the Allies move in.
Dragonslayer. The end of magic and dragons, and the start of Christianity.
The final third of Into the Storm (2009) showcases the post-war period. It has a very melancholic feel to it, displaying the definitive end of the British Empire and the downfall of Winston Churchill and the sort of Imperialistic, larger-than life politician he represents.
Jurassic World: Fallen Kingdom: The final remnants of Jurassic Park/World are about to be completely destroyed in a cataclysmic volcanic eruption, which will wipe out the remaining dinosaurs once and for all (we're not really told what exactly happened to the other islands). The plot revolves around the characters trying to save a small number of them from extinction and by the end InGen's monopoly on cloning technology and the dinosaurs isolation from the mainland also comes to an end.
The Last Samurai and The Hidden Blade are both about the end of the samurai age. In The Hidden Blade, an expert in western culture even teaches the samurai how to run in the "western style."
Nicholas and Alexandra: The film dramatizes the fall of Tsarist Russia. Count Witte describes it in such a way.
Count Witte: None of you will be here when this war ends. Everything we fought for will be lost, everything we've loved will be broken. The victors will be as cursed as the defeated. The world will grow old, and men will wander about, lost in the ruins, and go mad. Tradition, restraint, virtue, they all go. I'm not mourning for myself, but for the people who will come after me, they will live without hope. And all they will have will be guilt, revenge, and terror. And the world will be full of fanatics and trivial fools.
A recurring motif in Ocean's Thirteen, various characters remark at different points at how the casinos and heists in Las Vegas have changed around them ("You're analog players in a digital world"). The changing of the times also divides the crooks of the setting between the heroic Gentlemen Thieves who abide by the codes, and the villain of the movie, who sees the modern Las Vegas as an excuse to betray it.
One of the themes of the first Once Upon a Time in China, lamenting the fading of Kung Fu in the face of modern weapons. Particularly exemplified by Anti-Villain Master Yen. Furthermore, the increase in westernization among Chinese people emphasized on how the characters deal and cope with it.
All in one film, Pacific Rim covers the end of the Kaiju-free Earth, end of the Jaeger's golden age, and lastly the end of the Kaiju era...perhaps.
James Cameron's Titanic (1997) can arguably be this, especially for the viewpoint of the Present Day Rose.
The Serbian film Underground is about the dissolution of the united Yugoslavia, which the film mourns. Many critics did not appreciate the film's romanticizing of Tito's communist regime.
Who Framed Roger Rabbit? links the construction of the Pasadena Freeway to the end of old-timey Los Angeles culture, represented by The Golden Age of Animation come to life. For anyone who knows the history of LA, that was actually somewhat Truth in Television. It's the freeway that was a major factor in the basin's development and subsequent suburban sprawl. However, the freeway was actually built seven years before the film is set, making it strange that the very idea of a "freeway" is presented as a novel concept that everyone but Judge Doom thinks Will Never Catch On. Also, living cartoons never happened.
Discussed in Yamato. Kamio, being one of the surviving crewmen of the eponymous battleship, has spent the past few decades mired in Survivor Guilt, unable to move on. By recounting his tale to Makiko, he is finally able to acknowledge that the Showa period has come to an end and let go of the past.
Daughters of the Dust: Peazant family matriarch Nana fears that in leaving the island, her family will forget its Gullah roots and become assimilated.
This trope is invoked by name in the Dutch film Admiral, which partially takes place at the beginning of the end of the Dutch golden age. At the funeral of Michiel de Ruyter, the eponymous admiral, one of the aristocrats mentions his death heralds the end of an age. See Real Life - Early Modern for more information.
Avengers: Endgame does this for the first twenty-two films of the Marvel Cinematic Universe, retroactively known as the "The Infinity Saga." By the end of the movie, the original six Avengers are no more. Tony and Natasha have sacrificed their lives to defeat Thanos, Clint once again retires and reunites with his restored family, Banner's arm is (possibly permanently) damaged from using the Stones, Thor leaves Earth to join the Guardians of the Galaxy on their adventures after deeming Valkyrie the ruler of New Asgard, and Steve, after going back in time to return the Stones to their original locations in history, chooses to return to 1940s to live a full life with Peggy.