Captain America's "Snap" Sacrifice Came 8 Years Before Iron Man's
Avengers: Endgame saw Iron Man do the final snap to defeat Thanos, but Captain America already pulled a similar move eight years before this all played out on the big screen. Marvel Studios had their work cut out for them with the 2019 Joe and Anthony Russo film. Aside from the pressure of sticking the landing for the MCU's Infinity Saga, it was also expected to bring a satisfying closure to Tony Stark's and Steve Rogers' respective arcs. While the heroes' stories wrapped up very differently, both of their final fates made sense.
That being said, it doesn't mean that there aren't questions about how the Russos, alongside writers Christopher Markus and Stephen McFeely, decided on their narratives. There's still an argument over whether or not Iron Man should've survived Endgame, while Captain America's decision to go back in time and be with Peggy contradicted his earlier stance in the film. Aside from that, there's also the idea that Steve making the sacrificial play made much more sense considering his unwavering commitment to his responsibility as a superhero.
In hindsight, however, Captain America sacrificing himself wouldn't be anything new as he already did his version of Tony's snap long before Thanos' presence in the MCU was established. Steve already made the same heroic play that Iron Man did to ensure the Mad Titan's defeat, albeit arguably in a less dramatic way in Captain America: The First Avenger eight years before Endgame. Toward the end of the Joe Johnston film, the Super Soldier decided to crash Johann Schmidt, a.k.a. Red Skull's plane full of weapons of mass destruction after finding no alternative way to land it without detonating them and causing a catastrophe. While Howard Stark attempted a search and rescue operation for Steve, they were unsuccessful with this endeavor, concluding that Captain America was dead.
Like Iron Man, Captain America's decision was motivated by putting the greater good ahead of his safety, both of them giving up a future full of potential. In the split second Tony decided to use the Infinity Gauntlet to defeat Thanos, he also effectively chose to make peace with Pepper Potts and Morgan having to move on without him. Similarly, Steve came to terms that he would never have the chance to have a life with Peggy Carter in The First Avenger. The only difference between these instances is that Captain America miraculously survived his big sacrificial play, offering him a another opportunity to reclaim what he lost when he crashed the plane in the '40s.
This is also one of the reasons why Captain America enacting the final snap in Avengers: Endgame wouldn't have the same impact as it had when Iron Man did it. Despite the specifics being different, Steve sacrificing himself for the greater good would be nothing new. It's expected of him and frankly would have felt repetitive considering what he'd already done in The First Avenger. Seeing him finally get the happy, quiet life that he deserved with Peggy was a more fitting ending for him.