Originally Posted by Spider-Fan
No, it is because it is better from a writing standpoint. Her being dead is sad, yes. But, you don't see the contrast that an elderly Peggy does. When you see Peggy old and gray, you SEE her journey to where she is now in contrast to Steve's. You also see a human reaction to her finding out the man she loved, who she thought dead, is alive. And he doesn't look any different. It's far more sad and emotional if she is still alive.
Probably the best explanation and reason to include Peggy in the sequel. I think you need her to sort of help Cap complete his character arc that Marvel set up in TFA.