Question: Without the fiat of the Blessed Virgin Mother none of us would have the incomparable beauty and truth of Our Lord Jesus Christ incarnate! If one needs proof of this, simply look at how the archangel Gabriel was sent from heaven to ASK Mary's permission for the Son of God to be incarnate in her womb!
Answer: I took your excellent suggestion and looked in the Gospel of Luke. I did not find Gabriel asking Mary's permission, but informing her thus: "Do not be afraid, Mary, for you have found favor with God. And behold, you will conceive in your womb and bring forth a Son, and shall call His name JESUS."
"You will conceive" is not asking permission but a statement of fact.
The incarnation of the Son of God and the redemption He accomplished was not dependent on human will but on the unchanging plan of God the Father. In His eternal and immutable decree God prepared willing servants like Mary just as He prepared reluctant ones, like Zechariah, and even evil and wicked ones like Pilate and Annas, to achieve His eternal purpose.
Question (2) But "you will conceive" is an example of God's divine courtesy. It is the very essence of love to ask permission to do anything. God never forces Himself upon us. We must always accept His invitation to love. Otherwise it would not be love but merely coercion. The Blessed Virgin Mary could have certainly refused to accept this divine invitation. So yes, the Blessed Mother could have said "No" to God and therefore the statement "you will conceive" would not have come to pass!
Answer What if Mary said "No"? You fancy that God's plan would have come to a halt, and the incarnation would not occur. But that is a foolish thought! God does not have to use coercion to accomplish His unchanging plans. He Himself had predestined Mary for that very purpose, that she should willingly be the mother of the Lord according to the flesh.
In the same page we read how the angel also told Zechariah that his wife would conceive and have a son. Again the angel was not asking permission. Unlike Mary, Zechariah did not believe. What happened? Did that alter God's plan? No, not one bit. Zechariah and Elizabeth still had a son, John, as determined and planned by God.
God orchestrated the history of salvation according to His
eternal plan. We should therefore look to Him in adoration and with
everlasting gratitude for the redemption which was planned before the
world begun and which was perfectly unfolded by the invisible hand of the
sovereign and almighty God.