What's the Point of Mary's Perpetual Virginity?

Protestants often balk at Catholic devotion to Mary. But that devotion is there as a teaching aid. In general, Mary's exalted position speaks to the majesty of her Son. What could a little girl add to the majesty of the Son of God? Only that he could not be contained, or born from, a lesser vessel. Much has been written on this, and I won't add to that,

What I recently realized though, was what Mary's status teaches us about us. You see, if Jesus must have been born by a pure, that is, sinless vessel, then Mary would have to have been sinless.  But her status must have continued throughout her life, not just until the birth of Jesus.  Why her whole life?   Because if she ever sinned, she would have been a sinner, just like us.  That is, we sin because we are sinners, not the other way around.  Mary remained sinless because she was not a sinner.  Thus, the Church claims she is ever virgin, and that she was assumed into heaven (her assumption speaking of her perfection at the end of her life).

And what about us?  We will be sinners until the end of our life, not matter how much God may purify us in response to our prayers.  Humiliating, isn't it?