First off I think that C.S. Lewis did a fantastic job on the books and I love them all dearly but I also love the movies just the same. I feel like allowing Susan to have a short romance with Caspian adds to her character. After Prince Caspian Susan became a silly girl. She stopped caring about Narnia and focused her energy on superficial things like dates, and make-up and, clothes. Yet why would that be? Why would Susan who was always viewed as the sensible one make this sudden change in her personality? The answer is very simple, she became heartbroken.
In the books the fact that she couldn’t come back to Narnia broke her heart, in the movies it compounded by never seeing Caspian again. It seems like Susan spent all her energy on ways to forget about Narnia and all that had happened there because she couldn’t bare to live without it. When Lucy and Edmund came back she was jealous of them and when she found out that Caspian had gotten married she was finished. She pretended like it was a silly kid’s game and whether she believed that it really was all in her head or not she denied its existence to keep the pain away. She abandoned her inner life to live a surface life because dipping below would mean that she would have to admit to her pain and deal with it.
Before C.S. Lewis’ death he mentioned that he was planning or had started writing a book about Susan and her story. We never did learn what truly happened to Susan so that leaves it to the reader’s imagination. This is great from a novel standpoint but for a movie you need a bit of closure.
For the wide audience that Narnia was trying to appeal to many might not understand the feeling of the rejection of ones past religion which the Narnia books were eluding to yet the feeling of leaving behind your first love is something that is more relateable in the surface medium of movies. So I think this is was Andrew Adamson was going for.
I think that both sides work. In the books the platonic relationship Caspian has with the children works, and the romantic relationship he has with Susan works for the films.