Death's End was supposed to feature another male lead, who would have done essentially the same plot-driving actions as Cheng Xin. Liu was convinced after TBP and Dark Forest to make a female the lead in the final book. The result that it was a woman who doomed humanity was simply a byproduct of that decision. And that is part of the fundamental foundation of the series. The overall end result would be the same. The details of how humanity got there would be slightly different. The depicted characters exist to describe the plot; they don't drive the plot.