I think its a very emotional fan-fic, yeah many times I was sniffing and even that we all wanted to happy ending Im afraid that I knew how it was going to end from the very beginning...I mean, "She comes from a different world" and yes she does, so we can throw away the "Happy ending" teory since she belongs to her world as we belong to this one.
Plus if you ever read something about travelling between universes, any kind of foreign object introduced to another dimension can unbalance the delicate flow of time (Insert Doctor whooves quotes maybe?).
PLUS, I think that the end was thought for the ones that rather being happy for what they`ve lived rather than living something so fantastic...
So sorry to break the bubble