With Robert Downey Jr. bowing out of the Marvel Cinematic Universe in Avengers: Endgame, the film had to find a satisfying way to write Iron Man out of the series. It did so by having Tony Stark use the Infinity Stones to defeat Thanos once and for all, sacrificing himself in the process. But as it turns out, it shouldn't have ended that way.
While Tony Stark's death worked on an emotional level, showing his character growth since his first appearance in 2008's Iron Man, the logistics of his sacrifice were always going to receive scrutiny. It's difficult to accept a character like Stark not having a way out of any situation, and that's especially true given how he interacted with the gems before the events of Endgame.
Iron Man's Resistance To The Infinity Stones in The Avengers
In 2012's The Avengers, Loki attempts to mind control Tony with the Mind Stone. But as Loki taps Tony's arc reactor, it has no effect, with the god of mischief instead having to toss his opponent out of the window in an attempt to eliminate him. While being able to withstand one gem isn't the same as being able to handle all of them, it points to a deeper potential.
Stark's resistance to the Mind Stone likely stemmed from the new element he created to power his arc reactor in Iron Man 2. Needing the new element to save his life, Tony uses Project Pegasus' resources for help, the same organization which was created to wield the Tesseract's power. Beyond how Tony could resist Loki's scepter, using the Tesseract as his power source would also explain how he was able to survive his first encounter with Thanos on Titan in Avengers: Infinity War.
Iron Man Should Have Survived After Using the Infinity Gauntlet
From Iron Man 2, Stark's arc reactor has been powered by the Tesseract, which means his armor should've been imbued with the power of an Infinity Stone with every subsequent iteration. And if that was the case, he should have been able to withstand the Infinity Gauntlet's deadly cosmic energies, just like The Hulk did. Hulk was able to survive because of his exposure to high amounts of gamma radiation, and if Tony was living with the power of the Tesseract lodged in his chest, the same logic should apply.
If Iron Man was empowered by the Infinity Stones, using the Infinity Gauntlet shouldn't have killed him. Instead, he should have been able to complete the circuit and become one with the stones' combined power. He should have been immune, or at least resistant, to the worst side effects of using the gauntlet, just like how he was able to resist Loki's attempt to control him all the way back in The Avengers.
However, while the MCU teased that Iron Man was powered by the stones, his story was always going to end with a sacrifice play. Unlike Captain America, Iron Man's story couldn't end with an idyllic retirement with the love of his life. Regardless, it might bring comfort to some fans still mourning the MCU's original hero, wondering whether there was a way he could've survived.