Someone on this board actually argued that the whole line “Once a King or Queen of Narnia, always a King or Queen of Narnia”* refers to the idea of Eternal security, and that there was no way Susan would not come back. As you pointed out, the reason she didn’t die was because she would not have been ready.
I’ve always felt sorry for Susan, because I don’t think there’s anything in the previous stories that warrants her not being with the rest of them. It’s just that Lewis needed some person to stay behind, and it couldn’t be Peter, the leader, Edmund, because he’d already screwed up, or Lucy because she consistently had the faith of a child.
And, yet again, I take the topic so far that it should probably be a separate thread. If people show interest, I’ll start one.