The following contains big spoilers for Avengers: Infinity War, in theaters now.
At the end of Avengers: Infinity War, the Mad Titan Thanos makes good on his promises. All six Infinity Stones in his possession, he snaps his fingers, and we watch as half the universe crumbles into ash and disappears.
Bucky Barnes, Falcon, Spider-Man, Black Panther, damn near all the Guardians and a variety of other characters just disappear to the horror of those that remain who are helpless to do anything. It's a powerful, gutsy move to end the movie on, with the strings rising as Thanos smiles in satisfaction at what he's done, relaxing in a hut and looking at the sunrise. But for as many fans as got choked up at the various deaths throughout the film, there were others that called the move cheap and manipulative, convinced that none of these deaths would actually stick by the end of the sequel next year.
The Marvel Cinematic Universe has long been criticized for its lack of stakes and willingness to commit to death. Like the comics they're based on, death isn't so much a permanent thing that happens as it is a temporary stopgap. Groot dying in Guardians 1 was essentially reversed 10 minutes after it happened; Nick Fury didn't actually get killed in Captain America: The Winter Soldier; and most famously, the death of Phil Coulson in The Avengers was reversed with the spinoff series Agents of S.H.I.E.L.D. And in the leadup to Infinity War, there were plenty of fans that wanted there to not just be deaths, but for them to actually stick.
Plot immunity for characters is nothing new, especially for comic book and sci-fi characters; Superman died but came back in Justice League, the door is open for Luke Skywalker to come back in Star Wars via the Force, and so on. With Infinity War, it's not exactly a coincidence that several of the characters who get ashed are ones who have sequels that have been confirmed in some capacity. Spider-Man's sequel starts filming this summer, the Guardians will a third film in 2020, and Black Panther obviously has a future ahead of him. With Doctor Strange being the exception, Marvel Studios has been upfront about the future exploits of those characters.
But these deaths, however temporary, are not only not cheap, they actually worked to heighten the expectation for Avengers 4.