The Marvel Cinematic Universe would be very different if Iron Man had died in Avengers: Endgame, which would have had a huge ripple effect for the future of the MCU.
In the final moments of The Avengers, Loki's army seemed unbeatable, and the World Security Council fired a nuclear missile at the city of New York. In the heat of battle, Tony Stark carried the missile through the portal, detonating the enemy forces and saving the Earth, but potentially sacrificing his own life in the process as he fell unconscious back through the closing portal. For a moment, it seemed as though Iron Man paid the ultimate price, but luckily he was saved by the Hulk. Tony Stark would go on to play a significant role in many future MCU events, so if he had died during the Battle of New York, the landscape of the MCU would have been changed drastically.
Ultron Would Have Never Been Created By Iron Man
The next Avengers team-up came during the Sokovia Accords. If Stark had died during The Avengers, Ultron would have never been created since Bruce had his reservations about developing the machine. This would have had some major ripple effects on the MCU, specifically for the lives of Wanda Maximoff and Vision.
For starters, if Ultron had never been created, then neither would Vision. Vision was created as a synthetic body for Ultron's consciousness to inhabit, powered by the Mind Stone, but the Avengers would put JARVIS inside the new body instead, creating the hero seen during Thor's vision. This would also mean that the events of Doctor Strange in the Multiverse of Madness.
Many MCU Villains Wouldn't Exist (Or Would Be Very Different)
Throughout his tenure in the MCU, Tony Stark had the tendency to create his own demons, even stating so during the opening monologue of Spider-Man: Far From Home.
Even so, many MCU villains would have seen a potential increase in power after Stark's death. Iron Man 3 saw Aldrich Killian bring his Mandarin plans to fruition, with no Iron Man in the picture, there would have been nobody to stop him from infecting Pepper Potts with Extremis or killing the President. It's also a certainty that Stark tech and Stark industries would have been highly sought after by villainous forces in the days following Stark's death, a storyline that is set to unfold during the Stark's rival, Justin Hammer rise to become a much greater threat in the MCU, as he'd be the most likely villain to acquire Stark technology, that is assuming he could find a way to escape from prison.
Endgame Would Need Another Big Death
The MCU's Infinity Saga wrapped up with Tony Stark, who used the Infinity Stones to eradicate Thanos' army and the purple villain himself, sacrificing his own life in the process. This world-changing moment during Endgame would be hard to imagine with anyone else in Stark's position, but somebody else would have had to fill this role if Stark had died during the events of The Avengers.
Avengers: Endgame needed a major character's death in order to function as the climactic film of the Infinity Saga, and the only character who matches Iron Man's level of significance in the MCU is, arguably, Steve Rogers, a.k.a. Captain America. Rogers went through quite the journey alongside Stark during Endgame, ending the film retired as a superhero, and seen dancing with his lost love, Peggy Carter, in the past. However, if he had been the one to use the Infinity Stones and end Thanos' attack on Earth instead of Stark, this also would have tied up his story very nicely. This action certainly would have fit the character, a soldier willing to sacrifice himself in order to protect those around him, what better way to show this personality trait than to have his final act being saving the lives of everyone in the universe.
Spider-Man Would Have Been Very Different Without Iron Man
One of Tony Stark's greatest legacies in the MCU is the introduction of Peter Parker, a.k.a. Spider-Man into the Avengers family during Peter Parker's young Spider-Man would have simply stayed as an internet oddity without the influence of Tony Stark.
Even if Spider-Man had managed to gain more recognition as a superhero, the events of his solo MCU projects would have gone very differently. The Vulture likely wouldn't have been the villain of Spider-Man: Homecoming, but even if he had, Parker would have probably been seriously injured in his fight against Toomes without Stark's assistance, especially during scenes featuring the Staten Island Ferry. It's likely that Aunt May probably wouldn't have discovered Parker's secret identity, since it was the Spider-Man suit that Stark designed that he was found in. In fact, Spider-Man: No Way Home would have happened, especially since it's unlikely that Peter Parker would have even been aware of Doctor Strange's existence or powers.
The MCU Would Have Been Worse Off
Tony Stark did a lot during his time in the MCU, including bringing in new heroes such as Spider-Man and Vision, funding the Avengers between The Avengers and Captain America: Civil War, and saving the world (and universe) on several occasions. But Black Panther: Wakanda Forever, whose life as a hero is directly inspired by Tony Stark's heroic life.
Iron Man's MCU legacy is set to be a huge storyline in future Phases, especially with the release of Armor Wars, which will see Rhodey, a.k.a. War Machine, track down Stark's remarkable technology after it falls into the wrong hands. There is even speculation that Robert Downey Jr. could return to the MCU as Tony Stark during the Avengers: Secret Wars, since this story's comic inspiration sees many Marvel characters, new and old, converge in a monumental battle. One thing is for sure, if Tony Stark's time had been cut short during The Avengers, the Marvel Cinematic Universe would have been a much darker place, worse off without one of its most charismatic and entertaining heroes.
