I agree. They didn't have to make that the reason why Susan disbelieved in Narnia after that. Wasn't "she grew up" reason enough?
SPOILER ALERT SPOILER ALERT
Well, the way I saw it, it wasn't the leaving Caspian behind that made her not believe in Narnia anymore. The entire movie foreshadowed her downfall, and her reaction to Caspian only emphasized that. Here's something I wrote on fanfiction.net regarding the issue:
I also like how Susan was done, actually--it has surprised me how much and how well they've foreshadowed Susan's eventual decline that's described in LB--even beginning in LWW. The whole Caspian/Susan thing--at least, how she reacts to him...and to the geeky boy, (even the kiss), really, for me, and for a lot of other people it seems, highlighted her straying to the path of materialism. And a huge factor in that foreshadowing bit was the scene she has w/ Lucy right before Lucy has that dream of Aslan. That was a great job on the screenplay writers' parts--even the cinematography of the scene--how Lucy and Susan are shown lying far apart from each other--foreshadowed which way Susan will end up going. And it's amazing--they started foreshadowing all this right from the start in LWW, particularly when Susan steps out of the wardrobe for the first time and says "Impossible." Now, the only other person who says that in LWW is the Witch herself, when Aslan reappears after His resurrection. And, in PC, it's Miraz who says it when he sees Trumpkin. Notice, it's only the villains who've said "impossible" in reaction to the truth--and, ironically, it's Susan who first started the sequence of "impossible"s in LWW. I like the connection to the "dark side" that they've made there. I only hope screenwriter Stephen Knight continues that chain of "impossible" in VDT--it's something so simple yet so pivotal to the whole storyline.
and another:
Well, yeah, the Caspian/Susan thing bothered me too. But so much of it was cut out that it was actually forgivable...and given the context in which Susan was portrayed, I'm actually starting to think that the whole intention of inserting that "romance" wasn't solely for the purpose of having romance in the movie. Given that Susan, from the very beginning of the film in the scene at the magazine stand and her insistence in the train station that the 4 of them should accept living in England as opposed to continuously hoping for Narnia, is pushed toward a more shallow, "materialistic" (I wish I had a better word) direction, the flirting on her part actually made sense. She seems more concerned about leaving a possible relationship behind than with actually leaving Narnia and Aslan behind for the last time at the end of the movie. And, her interaction w/ Caspian serves as a comparison to her interaction w/ the geeky boy--she is completely cold to that poor guy b/c he's somewhat awkward and not exactly the dashing type, whereas w/ Caspian, she's immediately drawn to him b/c he's this new regal hero w/ royal status and is very handsome. Susan's decline in LB was foreshadowed so much in the movie that I was completely fed up w/ her character alone by the end of it. I highly commend Adamson, Markus, and McFeeley for great screenwriting on that aspect.
So, to me, it's almost as if she sees Caspian as a "type" not really a real "person". He's just...well...the Prince Charming, and the geeky boy isn't, and she chooses Prince Charming over geeky boy. When she leaves Narnia, she isn't necessarily so upset about leaving Caspian, but just the fact of having a "relationship" itself, I think, which further points her growing hold on materialism and superfluous things like status or physical attributes. After all, some people get into relationships just for the sake of showing off or being able to say "hey, I have a girlfriend/boyfriend" to whomever they meet. Remember, like I said in the first paragraph, I believe they started foreshadowing her downfall in LWW way before any of this PC romance thing was concocted. It only took one word of doubt--"Impossible"--for me to realize that's what I think they were doing w/ her all along. Hopefully I haven't bored anyone
SPOILER ALERT SPOILER ALERT
Anyhoo, BACK ON TOPIC!
I think an unknown or CAREY MULLIGAN should play the part of Ramandu's Daughter.
Carey is an AMAZING actress, and I think she'd be perfect. She's been in things like BBC's
Bleak House (Ada Clare) and
My Boy Jack (Elsie Kipling), and 2005's
Pride & Prejudice (Kitty Bennet). She exudes innocence and purity and everything I think Ramandu's daughter should be like just from her face.

And she isn't really that big yet, although she's had some main roles. And in some of the pics, she might look young, but she's actually 23.
Carey's imdb page and pics:
http://www.imdb.com/name/nm1659547/
http://i0.sinaimg.cn/ent/v/u/f/bleakhouse/U1305P28T17D5006F377DT20070112015225.jpg
http://kensforce.com/news061807e.jpg
http://newsimg.bbc.co.uk/media/images/40906000/jpg/_40906008_bleak_416.jpg
http://cache.viewimages.com/xc/78137080.jpg?v=1&c=ViewImages&k=2&d=17A4AD9FDB9CF193BBABBA93317D8B5F9DAB9EBB9F80F7EE284831B75F48EF45
http://newsimg.bbc.co.uk/media/images/40950000/jpg/_40950426_pairtwo.jpg
http://images.allmoviephoto.com/2007_And_When_Did_You_Last_See_Your_Father/2007_and_when_did_you_last_see_your_father_002.jpg
http://www.bbc.co.uk/drama/content/images/2007/04/19/carey_mulligan_396x222.jpg
http://i174.photobucket.com/albums/w120/jeffreyquah/careymulligan.jpg
And an article about her:
http://janitesonthejames.blogspot.com/2008/04/austen-actress-carey-mulligan-passion.html