I understand what everyone is saying but she isn't so much insisting on following the plot than protecting herself. Sure she can build affection, friendship, or even love with this person but it's got to feel different when you've read an ultimate romance novel of the likes of Romeo and Juliet, Orpheus and Eurydice, etc. you get my point where Cedric is proclaiming the ultimate soulmate passionate love for Adeline in book form. You've read about their soul-stirring romance of the ages. So why wouldn't you feel insecure about that, either taking that opportunity from someone or falling for someone who won't feel the same (or worse love someone else)? I believe you can love and build a fulfilling romance with most good people you come across even though there's surely a "better" fit for you elsewhere in the world. But there's a difference between that nebulous notion of mystery soulmate and say, somehow knowing an acquaintance of yours and your husband are perfect fits. I mean just think of it like Layla vs. Grandpa's actual wife. Grandma and Grandpa's relationship feels like a cute old married couple with built up affection. But as a young woman, I would not want to marry him knowing he still holds Layla dear to his heart as his ultimate soulmate.
There's really nothing wrong with setting a time limit. It gives both of them an out if the novel comes to pass and that out is basically the foundation of every modern relationship anyway. You make an active choice everyday you're together. If you aren't satisfied by that relationship, you move on