Santa is not a replacement for the Nativity story or vice-versa. All legends of Santa, of which there are several, start with the presumption that he came sometime after Christ and started his gift-giving career as a tribute to Christ, not as a replacement.
St. Nicholas is a religious figure who loved God, Jesus and children. In some stories, he gives gifts to children to honor the Christ Child, in some stories, he is send by Christ to bring joy to the children. Heck, even the Rankin-Bass cartoon says that he chooses to give gifts on Christmas day as a tribute to the gift God gave us in Jesus Christ.
Santa is NOT a contradiction to the Nativity story, but rather, a compliment to it.
Our perpetuation of the Santa myth is a tribute to the spirit of giving. A parent's giving to their children in the name of Santa is an example of unselfishness and demonstrates that giving without an expectation of recognition or gratitude can be rewarding.