Why would Amy Tan have Jiang fall from such Glory?

When I first heard that Winnie was going back to her father I though that Winnie's troubles could be over and that her father would protect her from Wen Fu. Then when the reader is again introduced to her father the reader sees that he is a shell of his former self and can do nothing to help Winnie. I think Amy Tan made Jiang unable to help Winnie to signify Winnie's helplessness in her marriage and how nothing can really go Winnie's way. Winnie's father is now unable to speak and is hated by all around him, but when we first met him he was so kind to Winnie.  Jiang actually appeared to want what was best for her. He was one of Winnie's few supporters in the tragic life she had. Since she has married Wen Fu nothing has gone her way and every time she tries to improve her situation she only makes it worse. I think that another reason Amy Tan made Jiang unable to help Winnie was that Winnie has to be the one to end the relationship and she can not have others do it for her. Throughout the novel we have seen Winnie always need to lean on someone like Peanut or Hulan. Winnie is never the one who makes the final decision. So If Winnie is to escape Wen Fu she has to be able to do it without any outside help.