In Catholic atonement theology, if God can save Mary from all sin without Christ, what was the point of Christ's death?

It is my understanding that in Catholic Theology the immaculate conception relates to the fact that Mary was born without the blemish of original sin. It is also my understanding that Mary was, through the grace of God, kept from ever committing any personal sin as well. As a Protestant, I have a hard time understanding how these dogmas do not completely negate the need for a savior for not only Mary but every human. If God can save one person from original sin and personal sin without the death of Jesus Christ why not everyone? How does the Catholic atonement theology address this?