If you know the basic contours of the short story, I would say that fate does not upset cherished plans. In fact, this is the very point that O. Henry is making.
On the surface, it seems like nothing goes correctly for Jim and Della. They are poor and even their attempts to get gifts for each other is foiled. Jim sells his watch to buy Della combs, and Della cuts her hair to buy Jim a chain for this watch. Fate seems to laugh at them! However, if we look at the deeper meaning of the short story, there is something surprising and beautiful.
Both Jim and Della give what they cherish for the sake of the other person. This act of self-sacrifice is what makes them wise.
The point is giving is more important than receiving. When we have this perspective, then fate does not upset anything. Here is how the story ends:
"The magi, as you know, were wise men--wonderfully wise men--who brought gifts to the Babe in the manger. They invented the art of giving Christmas presents. Being wise, their gifts were no doubt wise ones, possibly bearing the privilege of exchange in case of duplication. And here I have lamely related to you the uneventful chronicle of two foolish children in a flat who most unwisely sacrificed for each other the greatest treasures of their house. But in a last word to the wise of these days let it be said that of all who give gifts these two were the wisest. O all who give and receive gifts, such as they are wisest. Everywhere they are wisest. They are the magi."
There is no sense of loss.