At this point, Robert Downey Jr. can easily return to the Marvel Cinematic Universe as Iron Man, but that isn't the problem. As the MCU's inaugural hero, Tony Stark was the poster hero of the franchise. He was the most influential character of the Infinity Saga, so it was only fitting that his death in Avengers: Endgame marked the end of the MCU's first storytelling chapter. Aside from that, wrapping up Iron Man's arc by seeing his sacrificial play perfectly encapsulates Marvel Studios' knack for long-term character development, considering what kind of person he was when he debuted in 2008's Iron Man.

https://screenrant.com/robert-downey-jr-iron-man-mcu-return-problem/