The Avengers inadvertently found the secret to immortality in Endgame, and completely forgot about it. Here's how.
Time travel has always radically changed the universe of the movies it is introduced in, and Avengers: Endgame was no exception. After facing defeat against Thanos in Avengers: Infinity War, the heroes of the Marvel Cinematic Universe discovered the Quantum Realm. It was through the Quantum Realm that the Avengers were able to go back in time and retrieve the Infinity Stones. In the process of creating a time machine, they also inadvertently found the secret to immortality.
Species such as the Asgardians and Eternals were able to live many thousands of years, but they still aged over time. In Thor: The Dark World, Odin told Loki that Asgardians lived and died "just as humans do." A discovery was made in Endgame by Bruce Banner, however, that could potentially prevent people from dying of old age. While the MCU quickly glossed over the pivotal moment, it fundamentally changed the rules of its own universe. Here is how the Avengers found the secret to immortality, and then quickly forgot.
Before Tony Stark shared his Time-Space GPS technology with the Avengers, the team experienced many problems with time travel. Bruce's time travel tests involved pushing time through the machine's users, rather than sending the users through time. As a result, the test subject Scott Lang (a.k.a. Ant-Man), temporarily became an elderly man. Moreover, Ant-Man was also turned from a middle-aged man to a 12-year-old and, later, a baby. The results left the Avengers feeling disappointed, who either failed to realize or were disinterested in the fact that they had just successfully reversed aging.
Significantly, during his time as a baby, a 12-year-old, and an elderly man, Ant-Man retained his personality and memories. The Quantum Tunnel was able to change Ant-Man's age and physiology, indicating that it could be used by those who wanted to become young again. By repeatedly using the machine, the user would never age or experience human mortality. While the user was not made invincible, getting older would be something they would not have to worry about.
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Following the Battle of Earth, Steve Rogers traveled to an alternate timeline to live a life with Peggy Carter. He later returned to the main MCU as an elderly man and passed on the shield and mantle of Captain America to Sam Wilson. Steve presumably retired by choice rather than because of his old age. However, had he wanted to continue being an active Avenger, that option would have been available to him. Just as the Quantum Tunnel had de-aged Scott, the machine could have reverted Steve back to his physical prime.
The time machine was simultaneously an immortality machine, capable of eliminating a core part of the human experience. Doctor Strange revealed that the Ancient One drew power from the Dark Dimension to become immortal, something she would not have had to do if she used the machine. In Avengers: Age of Ultron, Vision expressed that "a thing isn't beautiful because it lasts." Yet, the technology used in Avengers: Endgame would give people control over the length of their lifespan. With the exception of injuries and health-related issues, the Avengers found a way to become invulnerable to the effects of time. None of the Avengers would be forced into retirement due to their age.
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The Avengers never claimed they were going to be able to save everyone. They have, however, done their best to save as many lives as they can. If the Quantum Realm was used by the Avengers to extend people's lifespans, that would raise all kinds of ethical concerns. Certain people would be able to change their age, while others would be denied access to life-saving technology. Since it would not be possible to save everyone, certain people's lives would be prioritized over others. Use of the technology could solve some problems, but at the same time undoubtedly introduce many new ones. Widespread public use would worsen global issues such as overpopulation and social inequality.
Masters of the Mystic Arts such as Doctor Strange and Wong have become well aware of the dangers of tampering with reality. They most likely would not look favorably upon the Quantum Realm being used to artificially extend lives. Many films in the MCU including Avengers: Age of Ultron and Spider-Man: No Way Home have portrayed unforeseen consequences arising from reality-bending magic and scientific advancements. As seen in Eternals, Arishem the Judge always intended for humans to eventually die, assisting in the birth of a new Celestial.
In real-life, Marvel Studios has likely ignored the implications of the time travel machine to avoid overcomplicating the MCU. Of course, there could be an easy in-universe explanation for why the machine has not been used to reverse people's aging. One could simply assume that the Avengers believe use of the technology carries too many risks, and will only selectively use it or have agreed to not use it at all. In the Marvel Comics and Fox's X-Men films, despite aging very slowly, even Wolverine was not immortal. Iron Man himself said in Avengers: Endgame that "part of the journey is the end."
A natural-born writer and storyteller, Matthew Kang has written for MovieWeb, Fan Expo, Buffer Festival, and several YouTube channels. His first experience at the cinema involved falling asleep during a screening of Disney’s Dinosaur.