Zia is a career 34% shooter on a high volume. That's not horrible, and accounts for her cold 29% shooting this year. She was at 40% last season. Not a player you want to leave open and take your chances.
I think Feagin could wait, since her and Watkins would be getting a lot of minutes once Boston/Amihere leave. Rivers knows that minutes will be available once Beal/Cooke leave as well. I think the current/incoming freshmen are less of a risk to leave than somewhat like Russell (who hasn't done much in 2 years, so I doubt they'd feel her loss).