I think you missed part of the story. The White Witch didn't outlaw Christmas; her magic was preventing it from coming. The breaking of her magic was that Christmas was coming. And as C. S. Lewis would have pointed out, who came at Christmas? To take Father Christmas out of the story would be the same as taking John the Baptist out of the Bible. They are both heralds. One of the things I didn't like about the movie was the way Father Christmas looked. Lewis described him fatter and jollier than the ones you see in our world. This guy looked like a grump just off the fat farm. There should have been more joy on the face of Asland's herald.
I see what you're saying but i guess what bothered me is that if Lewis symbolized a lion for Christ, then why not symbolize Father Christmas too? It just strikes me as a bit unbalanced in that regard.