You do realize that Winslet was in three Oscar nominated flicks before Titanic right? Sense and Sensibility won a few, and Winslet was nominated for best supporting actress. She was the winner of Empire's best British actress twice before Titanic.
Saldana was in Pirates, Star Trek and quite a few other films before Avatar, where you never even saw her face.
In other words, you couldn't be more wrong about them being unknown. Daisy was unknown.
And how do your explain away the amount of actual roles for women in these films?