It's not just about the abuse, though, it's about control. Sakura shows in HF that she's quite capable of opposing Zouken if it means enough to her (she refuses to control Rider despite expecting him to throw her to the worms for it), so why would she go for the Grail War if she didn't have him to force her into it. Further, even if he could get her to compete, there's no reason for her not to use Rider to free herself from him. And, even if she doesn't do that, why would she hand Rider to Shinji so he can use her to kill Shirou when she could just use Rider herself to protect Shirou? In canon, Zouken prevents that, because he'll force her to use Rider to win the war, but without the worms there is nothing to prevent her opposing him.
The only way to make it work is to drastically alter Sakura's personality, and make her a much weaker person. And, since what I like about Sakura is her mental strength, I don't like that idea one bit.