That Mary couldn’t be sinless because it’d mean she didn’t need a Savior.
“My spirit rejoices in God my savior.” (Luke 1:47)
God is the Savior of mankind and He won all graces for mankind through His death. He won graces for all men of all times and places, from the first man to the last man. He went down to limbo to lead the righteous there to Heaven after reopening Paradise’s doors for mankind, so obviously He was not inactive but active prior to the Death of Jesus Christ, giving men the graces they needed for their salvation and sanctification. And it is the same with Mary. Moreover, He gave her every grace, making her all-holy, because, as the Mother of God, she is also the Mother of His graces, for, God is Mercy Itself and so she who is His Mother is also the Mother of the gifts which He would bestow upon all souls, including her own soul. There is no limit to His Mercy; He can make a soul as holy as it pleases Him to make. But it requires cooperation, and not all souls accept God’s graces - not even the grace of eternal life. Mary gave herself entirely to God as His handmaid, she cooperated with Him her whole life and to such a degree that she is the most excellent exemplar of charity and hope; but such a degree of holiness, while “merited” by her by her cooperation, is actually God’s gift to her, for no one is good but God, only He can save us and make us holy. And if through the Sacrament of Baptism He can destroy original sin in the soul, why not exempt Mary from original sin? Original sin is not a part of human natue but is a consequence of man’s first sin; it is an unnatural and unfortunate corruption. God, though, preserved her from original sin, so she was not a fallen human being, and this by His grace, which He won for her by His Death and which she “merited” by her complete cooperation with Him. The Immaculate Conception is not impossible but is proper for Mary. If no one could touch the Ark that held within itself the Ten Commandments and live, than how could sin touch she who held within herself He who is our Lord and Savior?