Uh, why would you say that? You don't need Captain America/Steve Rogers around if you want to do ANYTHING with Sharon Carter. The only thing you'd need him around for regarding that is if you wanted to do anything else with their relationship. But yeah if they just wanted to have Sharon Carter on the show they wouldn't need Chris Evans to be around for that, just Emily VanCamp. I mean was Thor/Chris Hemsworth required the few times Lady Sif/Jamie Alexandear guest starred on Agents of SHIELD? Heck Chris Evans wasn't involved at all in the production of Agent Carter and Peggy Carter had wayyyyyyyyyyy more connection to Steve Rogers then Peggy did. And really Chris Evans wants to just end the arc with his character which is obviously going to end in the 4th Avengers movie anyway but if Agents of SHIELD is around after that or heck if Agents of Shield wanted to have Emily VanCamp in the show they still could. She still works for SHIELD and at most would just toss in a reference about Steve but it doesn't need to be any sort of focus with her character at all. You could really have Sharon in the show without that so... don't know why you said that honestly.