It's not just about marketability, it's that Natasha can serve the same role as Sharon, and it makes more sense because she's actually more different from Steve *and* she's already got an established relationship with Steve. It's almost the exact same thing as Stark making Ultron.
Yes, they could introduce a new kick ass SHIELD agent spy chick and build her up... but why when you have a perfectly good one that everyone already likes and respects right there. This is in addition to Maria Hill. So at what point do you stop making new kickass female SHIELD agents in skintight bodysuits using up screen time that can actually make the story better as opposed to merely accurate and just start merging them together?