In an interview with Empire, Anthony and Joe Russo explained why they went for a different ending that didn’t involve Captain America dying, and it had to do in large part with how the audience perceives the genre. Anthony Russo explains:
“We were saying to ourselves, the genre – and perhaps the MCU – has gotten to a point where the audience are sensing the patterns in the genre. Joe and I have always been about: how do we subvert genre?”
At the end of the movie, both Iron Man and Captain America live, but the relationship – not only between them but with the rest of the Avengers – remains fractured, leaving a somewhat open ending as it remains to be seen how the Avengers team will go on. Joe Russo explained killing Captain America was never planned for the movie, as it “just seemed like an easy ending” and instead went for a “more complicated ending” to “see the ramifications of that moving forward. Killing Cap ends that conceptually. There would be guilt on Tony’s part”. Anthony Russo sees the conflict as a “family fight”, adding that “the more difficult and more interesting place to leave a family fight is: can these important relationships ever be repaired? Is this family broken permanently?”