TV movies count also, btw.
Anyone remember the 1977 two part ABC tv movie titled The Trial Of Lee Harvey Oswald? It is the one where LHO is saved from the assassin’s bullet and goes on a long, drawn out trial, and on the walk back to jail, just before he is going to finally testify and the verdict reached…he is then killed by Jack Ruby and a note flashes on the screen that the makers of the film cannot provide the role of a jury and the final verdict is the viewers. Son of a bitch! The Trial of Lee Harvey Oswald (1977 film) - Wikipedia I remember that there was a large backlash from viewers, and justifiably so.
Your turn.
John Sayles’ Limbo might be a case in point.
Care to elaborate? In what way does it “cop-out”?
While I have loved the theatrical version of The Abyss since I was a kid, it really does cop out at the end. Ending the movie at “goodbye, wife” and him disabling the bomb would have been the brave choice. But then they had to have the aliens save the day and detract from the whole thing.
Deus ex machina has been a cop-out device for ages.
I know I know some movies that fit the OP. But I must have blocked them all out!
Will Smith’s I Am Legend. In the aftermath of a plague that turns humans into vampires, lone survivor Will Smith hides away in his New York apartment searching for a cure. At the end of movie, Smith discovers a cure and it makes its way to a small pocket of humans living outside the city. The book the movie is based off has a much sadder ending.
The same goes for Tom Cruise’s Edge of Tomorrow. It’s basically Groundhog Day except Cruise is fighting an alien invasion in this one. In the movie, Tom Cruise is able to alter the outcome of the battle to save someone. In the book, no matter what Cruise did, he could not save her. Hollywood couldn’t go with that ending and opted for the happy one.
My vote: the movie The Final Countdown, which I saw in the theater when it was released in 1980, in the base theater at Naval Station Alameda.
Spoilers for a 45-year old movie:
After building up to a supposed big climax with the USS Nimitz preparing to single-handedly take on the whole Japanese attack fleet, the time vortex abruptly reappears without explanation and the Nimitz is ignominiously returned to the present day.
After seeing the prelude with two F-14 Tomcats ordered to “splash the Zeros” (to which the audience full of naval aviators cheered uproariously in the theater I watched it in), everyone in the theater had the same thought as the movie headed to its anticlimax: “That’s it?!” 
Even at 12 years old I thought it was a crappy ending and a cop-out by the filmmakers.
I Care A Lot, saw it on Netflix. Cop-out Hayes coded ending which would have been better if the antagonist(s) had gotten away with it.
I adore Limbo. It’s the perfect ending for the story it’s telling.
Get Out was changed to a second, happier ending after test screenings hated the first one. Since they removed the policeman arresting him, it’s “cop out” in any sense. 
Every attempt at making a movie from this story misses the whole point. The world is all vampires now, they are the normal people. The protagonist Neville, is now the new boogie man who comes when you are sleeping, takes your life and soul. He is the new legend, the evil that comes while you sleep, even Neville begins to realise that. The world moved on and he is the new monster in the dark (or light), he understands why they fear and hate him, he accepts that.
But the movies are always about the last human on Earth fighting against the vampires.
He is the new evil, he is the legend to be feared, he is the one who needs to die. Matheson continues to roll over in his grave at what they have done to his story.
It’s still hotly debated whether the English dub of Spirited Away undoes the work of the previous 90 minutes by giving a final line to Chihiro: “I think I can handle it.”