I have to agree. After reading this novel in High School I came onto these forums proclaiming much the same as what you argue against. However having since gone through college and grad school, I've come to see that there's barely a moment when Fanny questions herself or her judgment. She questions what she would do if certain events would happen--such as if Edward married Mary--and "plans" how to respond to those situations if they were to arise. But that's what a survivor does.

Fanny has a survivor's mentality, mostly from how Aunt Norris has treated her at Mansfield Park. Fanny will do what she can to make do with what she has and survive. Having the luxury of living her life is not afforded to her, due to her station and position in the family. Therefore she can have no character growth until she is afforded the safety and security of a station in life. Until then she will do what she can to survive.

This is why Fanny becomes so valued at Mansfield Park at the end, because the rest of the characters (well, except for Aunt Norris, I'd argue--but she takes it to the extreme of super hardiness, and probably in the end is Fanny's best teacher on the subject of how to survive anything) don't know how to survive when faced with challenges. In the end the family simply trades Aunt Norris for Fanny as their "rock" to help them through troubling times (this is especially true for Sir Thomas and Lady Bertram). It's the supplantation of one kind of "survival"--the harsh, rough and hardy school that Aunt Norris symbolizes (one might even say it's an extreme of logic going into hyper-rational insanity)--for another: the quiet but heartfelt steady school that Fanny does best. Supplanting logical survival with emotional survival.

It makes sense that when Mansfield is faced with actual challenges and hardships (like Tom's illness and Maria's running away) they turn to Fanny, to be the rock to cling to through the storm, because she is constant and unchanging. Why is she constant and unchanging? I'd now argue that Mrs. Norris and Sir Thomas have artificially kept her in that way, due to the constant employment of the threat: "but if you upset us, we can send you back to Portsmouth at any time we like". Having that fear hang over Fanny's head will keep her from having any character growth because she will do what she has to in order to keep them happy and survive so that she can stay at Mansfield.

Survival, that's all that Fanny has been doing for the entirety of her life--and one could say the same thing of Aunt Norris, who continues to choose to "simply survive" despite being afforded the luxury of possibly questioning her own experiences. There's a possibility that Aunt Norris--being told off by Sir Thomas will have the potential for character growth, potential I say, but whether Aunt Norris chooses to take that moment to grow is completely up to her.

I will say, IMO, there is a potential danger for Fanny to turn into Aunt Norris, if she doesn't take an opportunity to grow at some point in the future. Hopefully an opportunity for such to occur will happen with regards to her relationship with Susan prompting her to have a moment of self-reflection as she sees Susan in the position she used to be in--with Fanny now a kinder and gentler version of Aunt Norris guiding her sister to be "just right" for the Bertrams. Hopefully, Fanny, having been in the situation before will have a character moment at that point, but where the story leaves off--I'm not at all sure she will, as she's been proven to be right thus far, she's in the danger of believing herself to always be right, which will eventually turn her into a softer quieter and more emotional version of Aunt Norris faster than anything else I know.