By any measure, Avengers: Endgame was the end of era that brought about the end for several characters and their stories. Most notably, Iron Man, the character that kickstarted the universe, died in the film's climax. Despite this, Robert Downey, Jr. is reportedly playing the character again in Black Widow, a prequel starring the other Avenger who died in Endgame.
Although they're not technically coming back to life in the present-day MCU, the returns of those heroes are part of a discouraging trend that's plagued the MCU for years. For as dramatic as they've seemed, numerous deaths in the MCU have been undone to some extent by later future projects. In Marvel's upcoming features and Disney+ series, several formerly "dead" characters will return in a capacity that cheapens the very concept of death in the MCU.
Back From the Dead
Two of Avengers: Infinity War's most notable deaths were Loki and Gamora. Both characters saw their deaths at the hands of Thanos during his quest to obtain the Infinity Stones. The time-traveling shenanigans of Endgame, however, effectively undid both deaths. For Loki, the actions of the Avengers in the past allow a past version of him to escape his custody at the end of the first Avengers, which allowing a way for a less friendly Loki to also escape his fate in Infinity War. It also sets up for the upcoming Loki series for Disney+.
By a similar token, a more prickly version of Gamora traveled with her adoptive father to the future before turning against him in Endgame, again undoing her original fate and making her available for Guardians of the Galaxy Vol. 3.
Another Disney+ show, WandaVision, is bringing back another dead Avenger, in this case being The Vision. This undoes his death in Infinity War, rendering it now entirely pointless. There's also the possibility that the show could bring back Wanda's dead brother Quicksilver, too. After dying tragically in Avengers, S.H.I.E.L.D. agent Phil Coulson went on to become the one of the lead characters of Agents of S.H.I.E.L.D. That's not even mentioning "The Snap," which killed half of the MCU in a dramatic plot point that was always meant to be undone.
From the looks of things, it seemed like Tony Stark and Black Widow's deaths are some of the few that will remain set in stone.
Death Is Just An Inconvenience
Bucky's death in Captain America: The First Avenger was also a huge part of Captain America's characterization in the first two Avengers films. Though his survival as The Winter Soldier is straight out of the comics, it speaks to how undermined the concept has become in both media for superheroes.
Ironically, 1992's "The Death of Superman" was a landmark for its idea of killing off such an iconic character. The event's reversal not even a year later cemented the idea of the revolving door of death in superhero comic books as a trope. Even to this day, the death of popular characters is cynically looked at as shock value and publicity, soon to be undone when fans ask loud enough.
The same thing could happen to superhero movies as well. Movies in the Marvel Cinematic Universe are already criticized as is by some for being safe, formulaic affairs. Fans will especially desire greater risks to be taken post-Endgame to retain interest in the studio's movies. The central drama behind superhero and other action stories is the danger that civilians and even the heroes themselves face. Viewers may stop caring if they to feel that any death or injury could be undone in some way or another.
Thinning the Herd
The Marvel Cinematic Universe seems to intend to chart new territory with Phase 4. Despite that, many of the founding characters from Phase 1 are still alive and (relatively) well. The best way to prove that death truly means goodbye for these characters would be to unquestionably kill one of them off. The biggest reason that death means nothing is that many of the same characters remain. Even the aged but still alive Steve Rogers at the end of Endgame allows for the character's return in some capacity.
If anything, the only characters that death means anything for are the villains. The aforementioned Loki and Thanos are some of the only villains to survive long enough for multiple films. This poor use of villains is another sore point for the MCU. It could also again lead fans to believe that there is no threat or potential for change in any of the upcoming movies. If Marvel still wants death to really mean something in the MCU, it would be best for the studio to let its dead heroes rest in peace.